Nice to see so many RnL folk here
Yeah, I've heard about the mumble feature in PR. Given PR's gameplay a feature like that really should have been implemented by default, I can see it being a lot of fun.
As for hearing enemies, I think you should be able to hear their voice at least, but it could always be done so that beyond a certain distance the treble level of the voice could be rolled to nearly nothing, so all your hear is sort of a faint, undiscernable mumble. It wouldn't sound too weird, it'd just sound like faint background chatter. At very close ranges you'd be able to make out what people say, but at that point it really wouldn't matter as you'd already be blasting each other.