The mumble overlay has been fully working since they implemented a few improvements a while ago. Make sure there are no other overlays conflicting with mumble.

How can I do that?

if you have -dxlevel in your launch config remove it

It is gone. These are my launch options:
-sw -noborder -w 1920 -h 1080 -console -novid -useforced

for me i have to make sure that mumble launches before tf2.

hope this helps

I've tried this :/ I don't know what to do :(

if you're using a cfg such as chris's cfg, delete "mat_antialias," delete that whole line.

....And if THAT doesn't work, go to Mumble, then to settings (make sure you check "advanced"), go to the overlay tab, go to "Overlay Exceptions" there should be two options: white and blacklist. Go to whitelist, then find hl2.exe from your steamapps -> tf2.

You mentioned the -nod3d9ex command, make sure you have it spelled correctly. Many people screw up the command, not sure if you still need it though. Worth a shot.

Where is the "mat_antialias" line?

Should be in your gfx config, but go to your cfg, then just ctrl+f "mat_antialias" in all your files.

You could try adding hl2.exe to your overlay whitelist in mumble.

My overlay also only works if I start mumble before tf2

I do use windows 8 if that makes the difference.