I don't know about LXDE but in XFCE this can be fixed with:
So finding the equivalent settings in LXDE might work. If your LXDE environment is using the xfce-mixer then the above commands should actually solve the problem.
xfconf-query -c xfce4-mixer -p /active-card -s PlaybackBuiltinAudioAnalogStereoPulseAudioMixer
xfconf-query -c xfce4-mixer -p /sound-card -s PlaybackBuiltinAudioAnalogStereoPulseAudioMixer
Essentially the sound mixer was not configured for Pulse Audio by default on my previous XFCE installation.