There is no sound after doing mute then unmute on XFCE 4 Ubuntu 14.04.
amixer command to see the status of the Master sound.
$ amixer get Master Simple mixer control 'Master',0 Capabilities: pvolume pswitch pswitch-joined Playback channels: Front Left - Front Right Limits: Playback 0 - 65536 Mono: Front Left: Playback 65536 [100%] [off] Front Right: Playback 65536 [100%] [off]
In above result the status of Front Left and Front Right is [off]. Meaning it is still muted even has been unmuted from the XFCE panel. Try to toggle the switch to make it [on].