No offence intended. One man's meat is another man's poison. If someone were to play/jam soft sentimental song in the bus, i would feel damn irrated. Some people like what you say might appreciatate it though, that i agree.
Point is, in a small congested environment like a bus or mrt...if someone were to start playing/jamming something regardless of what it is, his/her neighbours are effectively being forced to listen to it. If they like it, great...if they dont either they move away, endure it for the duration of journey or plug in their own mp3s. It is not like other public places where you can simply can walk off and that in my opinion is causing disturbance.
Anyway i dont see the point of doing this in public unless there are some special reason and surely there are better places to do this then on public transports.