Sound cues from companions in BG2 (especially in battle) are one of the most memorable things from the game for me. And since BG3 has turn based battles, importance of sound/voice cues from companions (and enemies!) is huge, as they are not only adding atmosphere and dynamism to the process but also providing additional feedback on what's going on on the battlefield and... emotions!
Currently in EA sound cues are too few and far between, so I'd suggest to add a wide variety of cues to the mix, as detailed below.
Voice cues & banter during battles - those DO NOT have to happen on every occasion, but rather from time to time, depending on the severity of situations, randomness, and maybe some other factors:
* on critical hit/miss
* on opponent's (attacker) critical hit/miss
* on critical hit/miss of other companion nearby
* on being poisoned/confused/chilled/etc or otherwise affected negatively
* on low/critical health
* on low/critical health of other companion nearby
* new enemy appear in visual vicinity
* performing sneak attack or other specialized attack
* character selection
* regular/default attack
* on opponent's death (after successful attack)
To imagine how fun it might be, you can find a suitable Minsk quote for almost all of the above :) Or try PoE Deadfire in turn-based mode.
Another important aspect: Spells & their casting.
While you can't use long incantations like in BG1-2 on actual casting (as it will be too long), you can use similar long incantations once you SELECT spell, before it has actually been cast!
Sound of such incantations should be much lower - almost like whisper or mumbling - kinda like you just quietly reciting the words before actually pronouncing them aloud.
That should definitely please the old BG2 guard :D
(more) non-combat voice cues & banter - also something to consider, as for now I don't feel that characters in party have enough interactions with each other, while not in camp.
Some ideas on when they can occur:
* character selection
* failed checks (investigation, perception, etc)
* (almost) all spell/abilities slots have been used - nothing or too little left for the next potential battle
* low/critical health
* performing lockpick/disarm or other specialized action
* going into sneak mode
Lack of sound cues and additional banter is something that bothered me in DoS2 as well. Hopefully, Larian can step up with this as they did with graphics and other aspects of the game.