I agree in parts with dark river and susu - I think for most hearing people it is seeing the signs accompany the music that is beautiful and if that gets people interested in sign and wanting to learn it then I am all for it. We have a group here called music for the deaf which run workshops on music with deaf children and also sign performances of musicals like CATS and Joseph etc. There is a big interest amongst the deaf where I live in songs being signed but my bug bear is when they are signed literally.
I go to great pains to make sure it is the meaning not the words thats put to music so that the 'dance' for the hearing has meaning for deaf people. When I teach songs in sign workshops at folk festivals I always get people to talk about what the words actually mean. I then give them some signs to chose from and show them how the change of sign can alter the meaning most people I work with are really intersted in this
Carols in churches here in the UK seem to be signed in Signs Supporting English ie. signs for keywords in English Word order - this is great for the deafened people I know as they are aware what the song already is but most born deaf people I know make no sense of this they sign along only when they have had a BSL explanation about what the song is about. The children I work with love music especially buffalo drums and big djembe because your bones vibrate with their resonance but the havent much interst in songs in sign.

By the way the best piano player I have known was profoundly deaf and later went blind through Ushers syndrome at which point she gave up on her piano playing - a real shame