I think voicechat is a nice idea, but there needs to be some controls to avoid the aforementioned spamming/abuse problems:
1. PTT (press to talk) button, else permanently open mics would be a nightmare, probably causing echoing and allowing everyone to hear one player's washing machine in the background.
2. Server side per-player voicechat disable, and a vote option to disable it for when the admins aren't around to silence spammers and other abusers.
3. Client side option to disable voicechat, as some players won't want to use it. There sould be a text announcement on the screen whenever somebody enables/disables it, perhaps an icon appears on the scoreboard too, to avoid people shouting "hello? HELLO? ARE YOU LISTENING?!" thinking a player has voicechat when they do not.
The other problem is that players without voicechat won't hear important messages while normal text chat gets to all players. I normally pipe the chat messages to a tts program else often messages scroll off of the screen before I've had a chance to read them.
Finally, I fully agree with this:
Why pack the engine with unnecessary stuff, first try to fix some important ballance issues and bugs, then think about new game-concepts and maybe in third line think about audio-com.
Just my £0.02