The most recent versions of this mod (past 0.2.2) break the music in Cracker's Wither Storm Mod.

Their mod is not open source so I cannot go look at the code. How is it broken?

EDIT: Try setting enableSoundPruning in the config to false.

Sorry for not responding sooner, I already disabled sound pruning and it fixed it. Strange but at least things are working now. The effect that muffles audio behind blocks also seems to affect the music from this mod, and the authors recommend disabling that. Is it because the music is auto labelled as ambient instead of music? Is there a way to fix it from your end or is it entirely up to them?

Yes - their music is submitted as "normal" sounds - AMBIENT, HOSTILE, etc. and is attenuated. If it is indeed MUSIC they should consider using the sound source/category of music, and set it the attenuation to NONE. My mod ignores the music category when it comes to special sound processing (reverb/occlusion). Sound pruning will ignore sounds with attenuation of NONE. (Attenuation of NONE means there is no volume falloff over distance.)