If you're talking about riding by yourself then I don't see the problem. Someone who passes you, or whom you pass, will hear a couple of notes. Same thing as happens when a car goes by you.

If you're with a small group, it's not really different from sharing a car or living room with them - there's a certain amount of mutual agreement in the volume and song selection.

Definitely they're annoying on a group ride ... but I guess I don't really understand why anyone would join a group ride if they just want to ride in their own little world. People do, though.