On PS5 at least, a bug like this has been there since forever. It sometimes affects characters selectively too - some party members will have most of their sound effects missing (as well as banter voice clips and combat voices) while others don't. As you say, restarting the game fixes it, but it returns later.
I haven't had it happen in Patch 8 so far, but that may be because I have to restart the game every couple hours regardless, because of the OTHER sound-related bug on PS5, the one where cutscene dialogue voice clips start playing too late, breaking most cutscenes once the bug has occurred. The devs are at least aware of the latter bug, but when I reported the bug you describe, the customer support person didn't seem to understand and thought it was the same bug as the cutscene one.
