Three of the more commonly-used music databases are Grove Music Online, Music Index, and RILM.
For immediate reference needs, you may wish to point the user toward one of these until he or she can follow up with the appropriate RSL.
Grove covers many musical topics and people, and can be a good starting point to get a general topic overview. Music Index and RILM are both EBSCO databases, and can be searched simultaneously for scholarly literature. RILM may be best for music history, musicology, and music theory, while Music Index may be more useful for articles relating to instrumental or vocal performance and music education.
Keep in mind that you don't need to include 'music' as a search term within a music database.
