try following few things :-

1. Go to acer web site... download machine specific drivers from their website... ie closest to ur model number...

2. under device manager (my computer -> hardware) ... get the model number and make of ur sound card and download the driver for the sound card... or Rt click on sound card and select upgrade driver... it might download new driver and make it work..

3. easiest might be just doing windows upgrade i.e. google .. windows upgrade.. it might
download new drivers (chances are pretty low).
lemme know if anything works...

good luck