I'm embarassed to say, I don't really get it. I'm sure you are right, but I can't quite wrap my mind around the reason.
Wouldn't the English translation be, "This piano makes a beautiful sound?"
Thanks again.
する is more close to ある in this case, not "The piano doessomething" but "Good sounds are in the piano"(sounds awkward, though).
This type of structure is often used in Japanese.
e.g.
頭痛がする(have a headache)
いい匂いがする(have a nice smell/there is a nice smell)
悪い予感がする(have a bad feeling)
The problem here is trying to translate directly into English. A lot of times it doesn't work and you should get used to grammar rules in Japanese not because it might make sense in English but instead because that's the way they are.
To my ears, を would make it feel like the piano is making sounds of it's own volition, which would be spooky and weird. What do our native speaking friends think?