I'm pretty sure this is a problem only for people HOSTING.
When I have more than 4 people in a game with my, the voice chat cuts out quite a bit for me and sounds a lot like the sound you hear when you're losing cell phone reception and you're talking to someone. It's VERY irritating and has forced us to use Skype.
The problem only seems to happen to the host and nobody else. No, its not an issue with my mic, it's clearly an in-game issue related to hosting.