On version 3.0.20 of the mod, for Minecraft 1.15.2, there seems to be quite a big gap when the loop restarts.

I think this might be what #44 is referring to, but I'm not sure since it's worded so poorly.

I just checked to see if the audio files loop correctly in Audacity, and they do, so I assume this is either an issue with the way you're streaming audio, or the way Minecraft streams audio.

It's an issue of the new sound engine. This issue will be fixed in 1.16: https://bugs.mojang.com/browse/MC-164435.