Recently smplayer (not mplayer) started behaving strange: every video started muted. Pressing the mute button (m key) really muted the sound, pressing the button again unmuted the sound and the audio started playing. This behavior persisted over restarts of smplayer.
Now that is really annoying!
Took me a while to figure out where this is coming from. My smplayer is storing the last configuration after every video (I like this feature), and somehow the config has a hidden option (one which is not revealed in the gui) for the volume, and this setting was 0.
The setting is in ~/.config/smplayer/smplayer.ini in the [%General] section:
[%General]
volume=0
Changing this value to something higher than 0 instantly enabled audio again:
[%General]
volume=25
And the new setting is persisted over restarts.