Open a terminal window by hitting: Ctrl+Alt+T
type: alsamixer
Hit F5 to show "All" audio channels.
Check if any show as [MM] i.e. muted. Pan to muted channel and hit M to unmute.
You could look for "Auto-mute" channel and disable
You could also have a play with "Loopback" disable or enable and see any effect.
If you find a working alsamixer setting, save it using:
sudo alsactl store
Sometimes sound problems are difficult and illogical to solve, so you may need to play around with settings and try different things.