pop menu item ”_Mute“ can not be translated

[SilverRainZ]

I am doing the simplified chinese translation for pnmixer, but I found the pop menu item "_Mute" cannot be translated.

In all the *.po file has string "Mute" but have no "_Mute", so add one can fix this bugs.

+ msgid "_Mute"
+ msgstr "_Mute"

I am no ensure that it is a correctly way, so I open this issue~

(forgive my poor english)

[elboulangero]

Thx :)