On an HP nc6400 laptop the mute light on the mute button stays on even when Ubuntu is not on mute, this only is happening after I upgraded to Ubuntu 11.04, it worked fine in 10.10. Even though the mute light stays on, if you start playing a piece of music or a sound it will play and the mute light will go off for the duration of the sound and then come back on immediately after the sound has finished. Pressing the mute button on and off only turns the light off for less than a second, it will immediately come back on again. The mute light will go off while I am adjusting the volume but will come back on no matter where I put the volume bar. While it doesn't affect usability it is rather annoying.
Try this: open a terminal and enter the following command:
gksu gedit /etc/modprobe.d/alsa-base.conf
Then add the following line at the end of the file and save it:
amixer set IEC958 off
The command above is to switch it off.
Now just reboot your system.