Voice chat: Incoming audio normalization

people's mics can be varying in loudness, and if i turn someone who's quiet up, they will then get distorted if they get closer to their mic later.

we could implement an optional normalizer on incoming audio to smooth things out by default. ideally this wouldn't be necessary as automatic gain control should be taking care of this for each individual, but it's inconsistent in its effectiveness, and some people turn it off when they really ought to have it on.

TeamSpeak 3 has this option, it's called "Automatic voice volume levelling" in the Playback tab. it's great!