Using Xubuntu 14.04 on a Intel laptop. Without pulseaudio muting/unmuting a single channel works normal. But when pulseaudio is installed:

• Sound Device: Card: (default): only Master channel is available which mutes and unmutes all channels, tested using alsamixer. At the end it works fine.
• Sound Device: Card: (HDA Intel): all channels are available here and if I mute a single choosen channel, it mutes all channels. And when I try to unmute it, it only unmutes that one channel and leaves all the rest channels muted. To unmute the rest I need to use alsamixer.

This is a suppose pulseaudio bug which happens when muting a single channel through alsa. The bugs can be found at https://bugs.launchpad.net/ubuntu/+source/pulseaudio/+bug/878986 and https://bugs.launchpad.net/ubuntu/+source/alsa-utils/+bug/1026331.

One possible solution would be to mute/unmute through pulseaudio when it's installed instead of alsa?