These databases will help you find articles primarily in scholarly journals; adding in Academic Search Complete will also help you find articles in newspapers and magazines.
This is by far the best academic database for research on music. It leans heavily to classical music, but the major academic journals on popular music are also covered.
This is another good academic database for music. You'll find a few things in here that you may not find in RILM.
Combine this with RILM for a more complete search.
Oxford Music Online is based on Grove's Dictionary of Music and Musicians, the gold standard of music encyclopedias, which has been in print for over 140 years. This is a great place to start your research because it will help you to get an overview of a topic.
Content sources are places where you can find audio and video recordings of a wide range of works. Mostly useful for classical music, jazz and world music are also well-represented.
Naxos contains an astonishing number of recordings. For classical music, there is very little you can't find here.
Another good resource to supplement Naxos.
Opera-specific, but includes video -- a must for opera.