ZDoom

Fixed. My hunch was correct; near limits were responsible, though indirectly.

I can confirm that looping is most likely the cause of this. Wads that utilize looping sounds for previously silent objects, such as decorations and in-flight projectiles in Doom Enhanced, can trigger this bug after maybe thirty minutes of hard gameplay, most notably with the plasma rifle (considering I borrowed some code from it before, and it worked in a previous SVN).

Now, I'm no coding expert, but it sounds like maybe the channels aren't being cleared properly.

Maybe it's the sound sequence. Or the fact that unless some custom stuff is loaded it's the only looping sound in the game. I suspect the looping is the reason why this is causing problems.

And it's not that sound lump in particular, because I've had the same issue while playing Hell Ground which replaces it.

This doesn't affect just this one sound. If it happens other things get garbled, too. It's just that the effect only seems to happen if this one sound is playing.

And it's really hard to reproduce with a freshly started game so I won't be able to produce results soon,

Can't test now, but what if you remove their near limits?

Yes, it is strange. I have now experienced it a few times and, again, always when a bunch of sectors were lowering after some time of playing. So, I made a quick test wad with around 300 pillars that all lower at the same time. I thought that would be bound to cause the problem but I can't seem to get the error with it - possibly because of doing it from a new start.

I got some more information. This apparently does not happen with a fresh start of ZDoom. It only happens after having played for a while. And I only seem to be able to get it when DSSTNMOV is playing, like when multiple sectors are moving at the same time. It sounds like the engine is running out of sound channels which is a bit strange because I already play with an extremely high number of them.

Yes, with r1976 and a savegame right in front of the yellow key, I was able to reproduce the effect 8 times out of 10 tries.

A place where I could reliably reproduce the problem is when getting the yellow key in Needs More Detail E2M6. At this point several pillars with monsters lower and a large number of sounds starts playing. My snd_channels setting is 128. When I set it to 256 the problem is still present but less severe.

http://www.doomworld.com/idgames/index.php?id=15280

I can't pinpoint the exact cause of this, only that it appears to happen most often when there's a few moving platforms (in particular stairs and crushers) nearby. Occasionally it happens that the sound becomes garbled for a few seconds and after that the sound of the moving sectors stops playing.

calling snd_reset or saving and reloading fixes the problem. I can't say when this started but it seems to be recent. I didn't experience this until a few weeks ago but since then it has happened several times already.