is it possible to install music that the person on the line would hear when you put them on hold?
It might be possible to write a dialer app to do that, but I don't think it's possible to just change some settings to do it.
It is not possible at this time. There are no apps for this function and to even write something that could possibly do it would be extremely complicated even if it were allowed. Stream audio to a phone call Android - Stack Overflow