Keyboard mute button not working in new user account

I created a new user account on my PC (running Win 8.1), and the mute button on my wireless keyboard does not work on the new account. Pressing the mute button causes "Mute" or "Unmute" to appear on the screen briefly, but the sound does not mute or un-mute. The volume up and down buttons work, and if I log into my old user account the mute button still works there. It only doesn't work in the...