This is the noise of a sad trombone, usually lowering in pitch after each 'wah,' that sounds off when someone whines or something bad happens. The last 'wah' sometimes is spelled with extra Hs or capitalizations, in order to drag out the last and saddest sounding 'wah.'
Interchangable with a fail, a boo hoo, or finger tear drops.
Jason: d00d that girl just shot me down.
Aimee: wah wah wahhh!
Jer: I wanted to go to the show tonight but then I got grounded because my mom caught me watching pr0n.
Craig: wah wah WAH!