This has been going on for a few years now, and I've found threads discussing it as far back as 2018. Very rarely Molag Bal's dialogue will play at dolmens. It does still show up in chat if the "NPC" channel is enabled, but it shows two lines instead of one for each time he should be talking. Examples from a dolmen in Alik'r:
These showed simultaneously just before the "boss" spawns.
And these just after defeating it. The doubled lines show even if Molag Bal's audio works. I did notice that if I ran dolmens in, for example, Alik'r for a bit, which each dolmen it became more likely that his audio would work, until eventually it was nearly every dolmen I could hear him.
@ZOS_BeckyIchnoski Could you pass this on? I hope my information was helpful.