Make sure you update the package lists first; "apt-get update" before "apt-get install"

I know it is there, because Debian says so.

EDIT: OH YIKES! For some reason, this VoIP client needs OpenGL support! WTF!? So, it may be unusable because someone thought it would be cool to use fancy OpenGL graphics. If the OpenGL portion of the app is in any way significant, you are out of luck, because the tablets don't do hardware accelerated graphics, and OpenGL is pretty much a no-go.

So you need to do:
This will force mumble to use software GL rendering, which will work, just incredibly slowly.

(Edit2: I'm hoping the OpenGL dependency is for linking into games, for the feature that will make the voice of the person talking come from their character in-game)

Edit3: It looks like they're using PulseAudio, which is also (currently) unsupported on the tablets. If you can't get mumble to use ALSA / ESD, you're out of luck too.

EDIT4: If mumble looks like a no-go, try the "teamspeak-client" package.