Loud music can be illegal and very dangerous as it blocks one of your main senses that should be getting used whilst driving, hearing. You cannot hear traffic signals such as train barriers etc when listening to loud music, so its not all to do with being a nusience, but I am pretty sure there is some law against the music being so loud for safety reasons anyway. Also, the louder the volume, the less aware you are, it hampers your alerness and reposnse times believe it or not.
Remember, it is illegal to wear headphones whilst riding a bike for the very reasons of audiable traffic signalling.
Common sense really, and I think it can make certain situations dangerous.