not only this but the steam/settings/Audio / Microphone control is "windowed" controled, so even IF you mute this to stop you talking ingame, it also MUTES your Microphone for Teamspeak.
Muting other people also stops them from hearing you, but there's no microphone mute button for some stupid reason - so basically you have to mute all other players to mute yourself.
Change the default device to another mic in the steam settings, so steam will not use yours. You need to mute everyone in game while in lobby, However once they are muted they will stay muted for future sessions, so for league racers this isn't a problem (even though all their mics are muted anyway)