And by that I mean, have a pop-up on the main page when a player visits the site for the first time (the same sort of pop-up as the ready up window.
Right now if I'm a new player to tf2center and I join a lobby I'll see this http://puu.sh/aCbnf/588f2bca7e.png next to a lobby and I have absolutely no idea what this icon is. So straight away there are new players joining mumble required lobbies who dont have mumble installed.
So I've joined the lobby and I see this http://puu.sh/aCbv9/a051f1c836.png and don't really bat an eyelid and just let the lobby start without joining mumble. From experience in many, many lobbies this happens when you ask a player to join mumble in a game only to be told "I don't have mumble installed". It can get pretty frustrating hearing it time and time again and it not changing.
I know there is the dropdown http://puu.sh/aCcoD/e92b30d33d.png but I think it is a bit late and possibly a bit obscure (clearly for some users anyway) But I think a little window when you open the site that says "note that some lobbies require you to be in mumble, a voice communication program which you can download here" with a little button that links you to the mumble.
This will catch new players eye a lot more and will increase the quality of mumble required lobbies.
Excuse any bad grammar its 1:50 AM and I'm tired but I thought this was a good idea.