I turn off any incoming or outgoing voice chat by default unless I'm explicitly playing with friends. There's the chance that I'm missing out on a nice chat with a stranger in the solo queue, but that's a rarity in my experience and I'm no longer willing to play social gaming roulette.
If players' voices coming out of your speakers freaks you out please DM me or thread for a story I'm doing. For example, your child has downloaded a seemingly benign game only for it to load them into an open mic with a strange man by default who now sounds like he's in your living room!