We have no final decision about this subject yet.
My personal opinion is: Let’s keep audio and printed works keep seperated. Recordings will be added to MB and printed works to BB, they can easily be crosslinked.
We would have thousands of dupes in both dbs, otherwise.
Think about another example. Many poems have been made into songs. But we won’t add this song-poem to BB. we add printed song texts to BB and the songs itself to MB. I think this is the best way.
Your book + CD contains both, the printed and the audio version. It should be added to both dbs.and be linked.