Front access panel button on Inspiron E1505 no longer functioning

For the last couple of days, the Mute/Unmute quick access button on the front of my Inspiron E1505 laptop no longer works. The button lights up blue and the QuickAccess dialog pops up on my screen with the volume meter, but the mute function will not turn on/turn off (depending on how it's currently set in Windows). The other buttons all are functional (Including the volume adjust buttons which move the levels up and down.

Please advise. Laptop is running Windows Vista Business.