Anyone who's sped up a voice recording and heard it turn into a squeaky chipmunk voice has run into the coupling between speed and pitch the hard way. It happens because naive speed change and true pitch-shifting are solving fundamentally different problems, even though both end up altering how a recording sounds.
Why simple speed change alters pitch
Audio is a sequence of samples played back at a fixed rate. Simple speed change works by literally playing more or fewer of those samples per second than the original recording rate, exactly like spinning a vinyl record faster or slower than it was cut at. Playing samples back faster compresses the sound wave's frequency, which the ear perceives as a higher pitch, and playing them back slower stretches the frequency, perceived as lower pitch, this is a direct physical consequence of how the playback rate interacts with the recorded waveform, not a separate effect layered on top.
How true pitch-shifting decouples the two
Pitch-shifting algorithms analyze the audio's frequency content directly and manipulate it in a way that changes the pitch of the sound without changing how many samples play per second, meaning duration and timing stay exactly the same. This typically involves breaking the audio into small overlapping segments, adjusting their frequency content, and reassembling them at the original timing, a more computationally involved process than simple playback-rate change, which is why dedicated pitch-shifting tools produce noticeably better results than just changing playback speed and hoping for the best. The Audio Pitch Changer applies this directly, useful for changing a voice's register for a specific effect, harmonizing with an existing recording, or transposing a musical piece into a different key. When speed genuinely needs to change, such as speeding up a podcast for faster listening, the Audio Speed Changer uses time-stretching techniques that adjust duration while keeping pitch natural, the reverse problem solved with similar underlying technology.