Experienced this last night:
I was in a party and we did a dungeon (via walking in at highsteppe, not matchmaking). Throughout the dungeon, and even after we left it upon completion, the audio worked fine. But once me and another member of the party left highsteppe he could no longer hear me, while I could hear him. I logged out and back in and the roles were swapped: I could hear him but he could not hear me. Continued re-logging seemed to just switch who could be heard. The only way to fix it was to leave the party and THEN log out and back in. After I did that, he re-invited me to the same dungeon party. Later (maybe 20-30 mins) he had an issue with his mount not activating and re-logged, only to have the same voice chat issue occur again. This time he and I both left the party and the audio worked normally for the rest of our questing.
Hope this helps!