That's happened to me: I either see
my Roland SC-88 tone module's instrument meters rising and lowering, or my E-Mu soundcard's doing the same, ahhhhggh! It's almost always invariably that you simply have a hard (switch on/of) or soft (volume control) break somewhere in your physical or virtual outputs.
First off, as obvious as this sounds, go over every piece of physical (hardware) and virtual (software) equipment you have that can possibly have any output fader or volume control. Ensure they are all engaged or (with a mixer) in (U)nity position. See if you get sounds from other sources you may have hooked up (e.g., FM radio, etc. if your final amp is e.g., a stereo receiver). E.g., your speakers ARE on, right? Also, if you have more than one headphone output, try them all. If you are getting sound to one source and not another (or others) this will help you pinpoint where the break is located.
Otherwise make sure that you sound-card and/or DAS/DAW/sequencer indicates you are outputting to the next source in the chain. The chain is important. Just ask yourself where the music goes in or starts, and follow the chain physically and in your mind in order -- piece by piece -- to see if everything makes sense or is logically executing.
But like I said, if you see the meters going it's really only a matter of some investigation to discover where the disconnection exists. I know, easier than it sounds, huh? Good luck!
