"I found out that a.tape2 is n.music on my BS9000;"
A.TAPE2 is the same as N.MUSIC - PHONO is N.RADIO.
"how can I manage to change the volume?"
In my setup I have a BS9000 - audiomaster in A.OPT 0, a BV7-40 MKIII - videomaster in V.OPT2 and BS as no audiomaster with Beolabs connected to BV7. I can control volume via BS5 wheel. I can start up the whole system by pressing GO on BS5, or turning the wheel on BS5. Now sources appear on the display and I can start up for example N.MUSIC by pressing GO. Standby ans system standby is also possible.
"And I always thought that I can listen to track "a" on the bs9000 (ie living room) and track "b" on the bs5 (ie kitchen)..."
If you are talking about listening to CD (BS9000) in the mainroom and at the same time listening to BS5 in a linkroom, that`s NOT possible. Masterlink is meant to distribute only one audio (stereo). If you have a Beovision, you can listen to BS5 in the mainroom via BV in PC mode and listen to BS9000 in linkrooms at the same time.
If you want access your BM5 music from a linkroom when you are listening to CD, just install Beoplayer on your PC or Mac. I have Beoplayer installed on my Macbook and my Mac mini - works great. See also my posts in the BS5 forum. Hope this helps.
Stefan