I've been using Dolphin to play GX (in non-ladder play mind you) at the likes of anywhere from 200% speed to 50% speed.

However, in any semi-recent version of Dolphin, the gameplay audio is tied to the gamespeed, so a faster/slower running game will also result in the audio being sped up or slowed down accordingly.

Luckily, there's a gecko code that can be used to alter the speed of the sound effects which can be utilized to cancel out the sped-up/slowed-down sound effects that you'd normally get when Dolphin is running the game at any speed other than 100% (note that doing this requires having time-stretching audio disabled).

XXX = 3F8 = normal speed
XXX = 3F0 = very slowly (very funny sounding announcer :'D )
XXX = 3F4 = Slowly
XXX = 3FC = Fast
XXX = 400 = very fast (announcer sounds like a chipmunk)

....but this only changes the sound effects alone and not the music.  Therefore I was wondering if there was any such code that could similarly alter the music playback speed?

And as someone with much audio waveform-editing experience, it's true that such a solution could be rigged up by exporting the music to PCM WAV and simply changing the playback sample rate (e.g. the "Set Rate" option in Audacity) and then re-converting back to GX's native music format and re-package the ISO, but not only would that be a bit tedious and would require going through the entire process for each different gameplay speed, but I don't even know if it's currently possible to convert audio into GX's native music format.