*BGM
Alt-O to bring up the dialog, make sure it's un-muted and the volume is up. Next, try running the setup program. I'm pretty sure you can control sound from there as well, though perhaps not the same way. That would usually be C:\Program Files\Gravity\RO\Setup.exe or something similar, depending on where your installation is located.
If those fail, you can also check if RO itself is muted (in Vista, you can control the volume/mute per-application, so that might be something to check), and make sure your sound card is actually working (are other applications able to make noise?)