HP 6550b, originally with Win 7, recently upgraded to Win 10, then the mute light would stay orange (indicating muted) when in fact the function worked as expected, showing in the apps. etc. what the actual state was: muted or not muted.
What worked for me: go to HP site, get the Keyboard driver (the light is on the keyboard!) for Win 7, (HP doesn't provide Win 10 drivers for this old machine!!) "HP HotKey Support", SP53796. I let HP Assistant do the installation, (where was this tool 30 years ago???) and it works.
Press the speaker light/switch to mute, it turns orange, press again, it goes back to cyan!!