chirp... that's the sound you hear every-time you press the Volume Up or Down keys on your keyboard. If you have the volume up listening to a song, the chirp comes out at the same volume - Not cool! Ever been in the Library and tried adjusting the volume? That's a surefire way to get some glares your way.
Let's fix it. Permanent solution: Turn it off in System Preferences
Open 'System Preferences', choose 'Sound'.
Now, make sure the 'Sound Effects' tab is selected. and make sure 'Play feedback when volume is changed' does NOT have a check-mark next to it.
Temporary Solution: Hold down the 'SHIFT' key while you are pressing the UP/DOWN volume keys.
Note: If you have gone with the permanent solution above, holding down the 'Shift' key while pressing the UP/DOWN volume keys will bring back the Chirp!