I know plex uses ffmpeg liberally to handle video and audio playback and transcoding throughout its products, and I’m guessing it’s the same with plexamp (specifically on iOS). Currently, with plexamp, if you change the audio speed of a song, it introduces tons of aliasing. I suspect this is because by default, changing the sample rate within ffmpeg uses a poor resampler that causes lots of aliasing.
The media player Cog on MacOS fully solves this problem using librubberband: https://cog.losno.co/
If plexamp could use librubberband like Cog does that would be amazing, and fix audio speed changes sounding terrible.
