Either I'm missing something painfully simple or everybody's mic is open all the time.

HALP!

Replying to myself because I'm a boss.

The most recent patch added F1 as the default push-to-talk key so maybe multi will be less of an open-mic nightmare.

I know you've already answered yourself now, but I just wanted to say: Assuming you have it on Steam, push-to-talk settings are a universal thing in all games. Also, you could just unplug the mic or get one with a switch on; it's nice to glance down at and check quickly when you get paranoid that people have just heard you singing to yourself.