UPDATE. Issue is back in 6.0. If a lot of combat sounds are going on at once and you are using a lower channel setting (such as 24) you are prone to losing mod sounds once more. :\
Something is seriously wrong with PlaySoundFile()
seems mods get very low sound channel priority now so our sounds get muted or clipped
DBM Problems Since Patch 5.3 - Forums - World of Warcraft
No sound notifications from any addons - Forums - World of Warcraft
Certain sounds cutting out - Forums - World of Warcraft
when sounds do play, they get truncated by game sounds and heavily clipped.
A patch was released to fix clipping issues
Many reports that addon sounds are being forcibly reduced in volume so addon sounds are much softer than they should be, so still not 5.2 behavior by any means. hard to hear over regular in game sounds, even when using master audio channel. so now instead of being unable to hear, they are just hard to hear unless you still turn game sound off and crank speakers up.