I find it surprising that TVB doesn't hold the rights to the music that they expressly commissioned for use in their own productions. They actually have to pay licensing fees to use their own series' music?
Stranger things have happened in the world of ownership and licensing laws, I suppose.
In any case, I have doubts that the master tapes of the BGM from 1970s and 1980s TVB productions even exist anymore. They were either wiped ages ago to make room, or have gotten lost or destroyed by other means over the years. I'd be surprised if they were still in pristine form somewhere.