This goes back to your other question of copyright. If you copyright your song, and someone else claims it as theirs, then you can pursue legal action (if you find out about it, of course). Don't just put the copyright symbol on your song and expect that to work. Fill out the paperwork from the U.S. copyright office (If you're in the U.S.) and send them the money (I think it's $20). Your copyright takes effect the day they receive your package, although you probably won't get your actual copyright certificate for a few months. A Google search on US Copyright Office should get you to their website (the .pdf forms are downloadable there).
Check out any site you may want to put your song(s) on. Email them. Voice your concerns to them, and see what they tell you.
What I'd suggest is to do something with streaming (if you can). This way, the file itself isn't on the user's machine, and the song is just streamed. There are various ways to do that.
Ideally, you would want to host your songs on your own website. If you have your own website and your own website address, then I'd consider finding a host that supports streaming, and use them.
Of course, one other much more difficult option would be to play your song through a Java applet (I wouldn't recommend this, however). You'd have to look up all the code and packages necessary to do this (they're all free), but it is time-consuming and difficult if you aren't a Java programmer.
Generally speaking, anything on the Internet and on web sites can be obtained if you know how.
