The wiper contact thingy (shiny) on the little volume board can bend outwards and loose contact, check the 3 wires going from the function (big) board to the mini volume board, push the wiper and see if you get a noise. Also, have you at any stage mistakenly plugged the speaker in to DP2 (Smoker socket)? If so, the speaker may of been damaged but rest assured another will be easy to come by.

If you're unsure on the speaker, find another small speaker to test, or quickly hook a small battery to the speaker and if it "thumps" (moves) if it does it should be fine to make noise.