Voice Changes (or Voice Alterations) have occured frequently in Inanimate Insanity. Whether it being a change in voice actor, or the voice becoming slightly higher pitched or lower.
MePhone4 using the old Christan Potenza voice.
Apple
While Apple's voice actor has remained the same, her voice has become much deeper and gravelly over time. It starts to become a bit more crackly in 4Seeing The Future, and in The Snowdown, becomes noticably lower-pitched at times. The transformation of her voice seems to have completed in Aquatic Conflict. Her voice is commented on being quite raspy in Marsh on Mars by Marshmallow.
Balloon
Balloon was one character who hasn't gone through a voice change, as his voice stays being the same high pitched one it always was. Throughout Season 1, all of Balloon's lines are recycled from his previous lines, starting in Sugar Rush. However, once Balloon joined Season 2 in Breaking The Ice, he talks once again, this time with a higher, and clearer voice. Once speaking in Marsh on Mars, Balloon's voice appeared to sound slightly lower, and now with a bit of a voice rasp. As of Cooking For The Grater Good, Balloon's voice actor changed to Ben Cross, giving Balloon a slightly altered voice.
Baseball
Baseball's voice became deeper once he re-appeared on Idiotic Island in A New Stage In The Game. His voice became even deeper in The Snowdown, as he pleaded the fans to vote for him. Towards the end of the series, Baseball's voice was raised a bit, and given more emotion.
Bomb
Bomb didn't necessarily go under a voice change, but rather stuttered less in future episodes. He mostly stuttered in the old episode A Lemony Lesson (excluding "Take the baton!" in War De Guacamole). He had a slight voice change temporarily in 4Seeing The Future.
Bow
Although appearing late in the game, Bow had a slight voice change since her debut. Her voice stayed the same throughout until The Tile Divide, after the hiatus, where her voice lowered a few octaves. As of her return in Let 'Er R.I.P. as a ghost, her voice is accompanied with an echo.
Cheesy
In the original advertisement, Cheesy appeared in One-Shot Wonder talking with a somewhat computerized high voice, similar to that of Paintbrush's, but when appearing in Sugar Rush, his voice even more computerized, and higher. As Cheesy joined Season 2, he first displayed his new voice in the "Cheesy Trailer", being much lower and less computerized than his previous version, yet still a higher pitched voice.
Knife
Though only slightly, Knife's voice became slightly deeper and expressive in earlier episodes. As the series continued, Knife's voice continued to deepen, and even develop a very crackling and raspy effect to it as well.
Lightbulb
While Lightbulb never actually went under voice change, her original voice actor was Christan Potenza, who also originally voiced MePhone4. Since her first line in A Lemony Lesson, her voice, like many others, deepened, but also acquired an expressive accent towards her later appearances.
Marshmallow
Marshmallow was originally voiced by someone named "Amanda" in The Arena Of Death. In The Stacker, Marshmallow received a new squeakier and higher pitched voice (done by AnimationEpic). The voice lowered itself ever so slightly throughout each episode, and took its final form in The Penultimate Poll. AnimationEpic, however, retired as Marshmallow's voice actor, and is replaced by DesuUagi for her new voice in Season 2, which is now higher pitched, clearer, and more feminine. It is even commented in Marsh on Mars that Marshmallow's voice is high enough to shatter glass.
MePhone4
MePhone4 has gone under arguably the largest voice change in the shortest time. He was originally voiced by Christian Potenza, voice actor of Chris McLean from the Total Drama Series, in the first episode The Crappy Cliff. However, immedieately starting A Lemony Lesson, MePhone's voice suddenly changed to a deeper, more adult voice, which started off nearly emotionless, but developed a character throughout the series, as early as The Arena Of Death. Besides this, MePhone4's voice has not changed at all from it's debut. Though, on occasion in Crappy Anniversary and Journey Through Memory Lane (Part 2), Christian Potenza will retake the role as MePhone4's voice, but this can be easily altered back by hitting MePhone4, preferably with a rock.
MePhone4S
Though only being a slight alteration, MePhone4S's voice was originally deep and mysterious in Sugar Rush. Once he returned in 4Seeing The Future, his voice did start off deep, but as he revealed his identity and intentions and progressed throughout the episode, his voice became angrier and meaner. Once returning in The Tile Divide, MePhone4S received a very Russian accent, based off of Arnold Schwarzenegger.
Microphone
Although recent, Microphone's voice became deeper, more expressive, and more tomboyish as the series progressed. It is also noted that when her volume is turned on, the gain has lowered quite greatly.
Nickel
Nickel went through nearly the same concept as Baseball, as the two both had their voices deepened throughout the show. However, Nickel's voice became much raspier throughout episodes, taking it's final form in The Great Escape.
OJ
Compared to his first voice role in The Taco Show, OJ's voice was originally quite raspy. Throughout the series, OJ was remarkable noted for deepening his voice dramatically, and soon even expressing much more emotion into the originally dull voice.
Paintbrush
Being only slightly recognizable, Paintbrush went through a very minor voice change as well. Originally, he/she was voiced by TotalDramaFantasy, but as stated in an Adamationz video, the voice actor "disappeared" for 3 months, until TheTGrodz began voicing him/her, starting in The Snowdown. As of this, Paintbrush's voice became slightly deeper as OJ did, and giving a very expressive new motion in Season 2. In Theft and Battery, her/his voice is slightly deeper and a bit less "scratchy".
Paper
Paper has gone through a minor voice change. Unlike most of the other characters, Paper's voice was lower earlier in the series. His voice became higher and more expressive as the episodes went by.
Pepper
Because of numerous voice actors taking her place, Pepper has had the most voice actors of any contestant. She originally started off with happy higher voice, her actor being Bridgetteandcody2 at the time, but once she appeared in Sugar Rush, she gained a new voice actor, Crasi4tunes, the same voice as Salt, though slightly higher pitched. However, her voice changed once again in The Penultimate Poll, as she then is voiced by Adam's sister, who also voices Salt, though keeping the same higher pitch. In Season 2, she changed voice actors yet again along with Salt, both now being voiced by AnimationEpic, but once again, her voice actor switched to Alexa Chapman, who voices both her and Salt, but keeping Pepper higher pitched.
Pickle
Besides having the voice deepen slowly over time, Pickle's voice has stayed virtually the same. In 4Seeing The Future, Pickle's voice became slightly deeper, but after the hiatus, Pickle's voice drastically lowered in The Tile Divide.
Salt
Salt's voice has changed slightly less than Pepper. Salt kept her voice actor, Crasi4tunes, up until The Penultimate Poll, whereas both Salt and Pepper were voiced by Adam's sister, having Salt take a lower octave than Pepper. In Breaking The Ice, she and Pepper were voiced by Adam, but in Everything's A-OJ, her voice switched to Alexa Chapman, giving her a deeper, more mature voice.
Soap
Like Microphone, Soap's voice started out high-pitched, but it became deeper and more expressive as the series went on. When singing, her voice is changed to Vibha Melkote, and her voice is much higher pitched.
Suitcase
Suitcase had a high-pitched, feminine voice at the start of the season, but from episode 5 on, it developed an Australian accent and gradually became much deeper and relatively raspier.
Taco
Due to the voice actor of Taco switching on and off between Adam and his sister, Taco has had voice changes all over, though only some are slightly noticeable. Adam first started voicing Taco in One-Shot Wonder, having his sister be the main actor before this. As the two switch between voicing her, sometimes both in an episode, the voice continued to get deeper, but will go very high pitched at many times. As of The Tile Divide, Adam became the final voice actor for Taco. However, when it was decided that Taco would make a surprise return in season 2 of II a new voice actor, Aceland Alexander Kent, was brought onto the team.
Toilet
Toilet's cameo in Breaking The Ice showcased Toilet's British Cockney voice. However, as the series progressed, Toilet began to shout more of his lines with intense emotion, further expressing his annoyance.
Tyler Bungard
When appearing in Double Digit Desert, Tyler had a higher pitched voice with a very strange accent, voiced by Justin. Once returning in Journey Through Memory Lane (Part 2), his voice was switched over to Fallingwithstyle1995, who had a normal-ish voice, now without any accent, yet still retaining similarly random changes in pitch and volume.