Actually they will not make any sounds. Everything will be just dubbing with your background mp3.

So something like Aliens will bounce/sing on your MP3 sound? (Beat detection etc?)

You will add mp3 and add tyrant for drums (for example). Then you will "program" him, so tyrant will hit drum in right time and it will looks like he really playing on drums.

If you just add players to scene and nothing else - they will stand there and that is all.

There will not be automatic beat detection.

Oh that's quite a cool idea
But also an option to lower/higher the pitch and making your own beats?

But also an option to lower/higher the pitch and making your own beats?

No, there is nothing what can affect sound. Maybe in Tremulous 3