@lolloo

That doesn't really refer to multi-tasking.
It's a feature built into the OS, when the phone functionality starts the software forces all other sources of sound/music to pause/stop/mute.
This is to convineance the user, so the user can initiate the call in the shortest time possible and also helps keep the voice quality higher.

And this function is built in, to pretty much, all other mobile OS's out there especially Symbian. I'm guessing the SE C905 chose to disable the feature, still not relating to multi-tasking.