The only decent way of including any form of voice over, in my opinion, is if it is optional. And by optional I mean it has its own audio track, seperate from all other music and audio, so when you turn it off it is simply not there. No music volume turned down for an absent VO (looking at you Come Home, one of the few detractors for that game), or any other audio shenanigans. It's fine if it's an option for those who want it, but I'll opt out every time.
Yeah, I'm not a fan of voice overs (in AVNs at least), could you guess? And honestly i can't understand why any dev would spend money, time and effort on that feature when the results are generally very poor. There are only a few VO artists willing to lend their talents for these adult games, so choices are pretty limited to begin with. Better to leave it out so you avoid players having the idea that a voice seems 'wrong' for the character, as is being dicussed here in the thread the last few posts.
(Edit: added 'time and effort' to money)