THIS.
I would say that, in my game, on average I get the tavern songs 40% of the time with vocals and no lute (instrument), 40% lute only and no vocals, and the final 20% with both as intended.
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Very interesting read - I have no experience with gamedev directly but what you say certainly sounds logical and reasonable.
Two questions, though.
1) Why would it only be happening for a few people (relative to the size of the playerbase)? From what you wrote it sounds like it should be fairly consistent in regular gameplay.
Unless of course the secondary bugs (like quests not being completed, leading to the sound issues) are the actually inconsistent part and once you get those, the sound issues are consistent.
2) If it's actually a memory issue caused by zone transitions, a pretty safe work-around for the sound issues should be restarting the game after every transition. Shouldn't be too difficult to test.
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Also, something I just remembered that supports your interrupt priority theory is that even without any banter some comments from my companions still fire if the situation if important enough.
With your explanation in mind it does make sense that you get no banter at all but when a dragon shows up, you kinda expect your companions to react as a player and they actually deliver.
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But whatever the cause may be, it's probably safe to assume that a second play-through is necessary to get the proper experience, cause all the banter we'll get after the patch will be referencing stuff we haven't actually heard.
In light of this, it's probably even best to finish the first playthrough before the patch comes out for the best experience in the second one.
It's a shame, though, cause there's just nothing like experiencing a game and story like this for the first time. And even if we get the banter later on, it'll just not be the same as having the full experience on the first playthrough...