I play classical music to my plants all the time and swear they love it. At times, I am sure they "ask" me to put it on!
Volume wise, if you stand where the plants are and you don't have to strain to hear it and it is not so loud that you can't hear anything else, is in the right volume range.
Loud enough to be heard clearly, but soft enough for conversation to be heard easily. Background level music.
I don't think they like opera or jazz though, but I believe they prefer piano music the best!
Hope this helps,.........
Organoman.