Is their a way to lock the volume envelope speed? I'm trying to replicate the volume envelopes of the original Gameboy but whenever I change the tempo it will either speed up or slow down the envelope.
Since changing the tempo changes the duration of a tick, that's technically simply impossible. However, you can try changing the ticks per row instead (Speed, Axx in IT/S3M) which can act as a coarse tempo adjustment.
Ok, will give it a try