How to find out what app is hogging the soundcard

[hollunder at gmx.at]

Does anyone know how I can find out which app is using or hogging the
soundcard? 
It already happens at startup. 
I can use audacious or mpd but not aqualung or jack. 

> Aqualung:
> Probing JACK driver... JACK server not found
> Probing ALSA driver... device busy
> Probing OSS driver... device busy

> Jack:
> the playback device "hw:0" is already in use. Please stop the
> application using it and run JACK again
> cannot load driver module alsa

I use ubuntustudio 7.10 from a fresh install + xfce.
It worked previously but I don't know what I did to break it.

Help is appreciated
regards
	Philipp