Will the BIOS codes beep thru your speakers?
When you start your computer, and hear a single beep denoting that everything is OK, does it come from the PC speaker or your other speakers? Some newer computers actually use the stereo speakers, but older computers use the PC speaker, a small speaker fastened inside the case.
Do you have any alarms set? Some programs, like Corel, have alarms. Check your tray to see if any of those can give you a hint.
Does it happen when you're doing something specific, like playing games? Some kind of system alarm, like a case intrusion alarm?
Are you sure you have the speakers plugged into the correct jacks? Do you have a setup program for the speakers with a test? (My ASUS motherboard does.)
Do you have stickykeys turned on? This is a feature that allows you to press one key and then it will hold until another key is pressed, but that's specific to XP, I think. It's enabled by pressing the shift key five times, and disabled the same way. You can check in Control Panel/Accessibility to see if it's enabled.
Does it beep all the time? If you shut it down for a few hours, does it beep again immediately upon restarting? (Could be the BIOS is trying to tell you that something's overheating.)