No, it's only global if set to be so in the server cvars. For some reason, quite a lot of publics do this - maybe its the default setting and turning on team talk only needs to be set up.
Bizarre!
I've also encountered llamas just playing annoying $~£# over their audio. You then go and mute them, and strangely within about 5 minutes they're re-enabled and you're get the annoying $~£# out of your speakers again....
ALSO, why isn't the mute option as easy as in TFC? Hold TAB, move your cursor over the speaker symbol on the score sheet and click... Done!