Well, I did some experimenting yesterday. It seems that the problem is the Beosystem 7000. The devicelist of the Beolink 7000 doesn't have sources like V.AUX or PC. So I think the Beomaster 7000 doesn't recognize commands like V.AUX or PC. The system does not know what to do with it.
The workarround I use now is first to give the command AV TV and after that AV V.AUX. Then the music files from Apple TV are played on the audio system.
An indication that the Beomaster 7000 doesn't know what V.AUX is, is that the Beolink 7000 shows on the (red) display source TV when AV V.AUX has been choosen.
Am I on the right track?
Piet