Hi Koert,
I don't know if you have the user manual or not. I've just looked up on page 42 for playing 'AUX' sources and it says that when using the A.AUX, though sound comes via the BS9000 and then in your case speakers attached to the BV7, it says that all control functions must be controlled from the device you are using - in your case the Airport Express. I can only assume that for whatever reason (I don't have one so don't know) the Airport Express is stopping the signal after 5 mins? Or, the BS9000 goes into standby as it's not detecting a signal from the AE?
From the BS9000 user manual:
Playing auxiliary sources
The A.AUX button selects extra Bang &
Olufsen equipment, you connect to the
BeoSound 9000.
You will then hear the sound from the
extra source through the speakers of
your BeoSound 9000 but all control
operations must be carried out on the
extra equipment you connect.
While the source is playing, you can,
though, use the functions which relate
to sound as shown to the left on the
page and the special sound adjustment
functions, please refer to the chapter
on sound adjustment on pages 40-41.
I don't know on your system if you can adjust any settings to prevent this?
Can you not connect the Airport Express directly to the TV and bypass the BS9000? Alternatively, direct to the speakers as described in the AE user manual?
I would have hoped that others with a simlar set-up, as several have Airport Express devices may have chipped in.
I hope you find a solution.
Dave.