The real reason that the music stops is the lack of the possibility to play sounds from more than one source at a time.

The camera application have to playback the shutter sound when your device is not in the silent mode (its a law in some countries) - the application can only do this when every other sound playback stops.

On the most desktop machines you will hear booth sounds at a time, but the N900 driver can't handle more than one source at a time. The driver can't mix it.

For the multitasking FUD: I can't playback a video in Background!
That's very sad. I would like to check eMails or write SMS sometimes during watching a movie. Stop the Image and continue only the audio playback would fix this for me.