I was playing cpd.wad and in the middle of MAP32 all monsters became mute, sounds could be heard only within a few meters from them even in the same sector. That level has commander keens and wolfenstein ss soldiers. Seems sounds were played either at full volume or not played at all. May be it's a problem with OpenAL itself.
I can't reproduce it even killing all the monsters and spawning new ones. It was LZDoom but Major Cooke has experienced it in GZDoom too.
It's rare and very hard to reproduce.