It is my experience that different sound sinks attached to a PC can cause different applications to use either sound sink ...... so some apps will give sound through the speakers while others might seem to be non-working, but most likely the sound is being sent to an unused sound sink, like the HDMI channel.
The solution I suggest (though most likely not the only one available) is to install Pulse Audio, enable it in PCC - Hardware - Sound and reboot the PC.
Then set up the sound system as you wish to use it, through Pulse Audio Volume Control.
To install Pulse Audio, select in Synaptic ...... task-pulseaudio and pavucontrol
PA Volume Control (pavucontrol) gives the user most all the controls s/he needs, on its various tabs.
For instance, I have the HDMI sound sink turned off in the Configuration Tab, so it can not be used by the sound system.
If I ever have need to use it, a simple click with the mouse turns it on again.
With more than one sound sink, each app can be told to use one and that setting will be remembered the next time the app is launched.
One of the sound sinks can be set as the default, so in the absence of an alternative, like USB connected speaker or headset, any app that previously had been using the alternative will use the default.
I like point and click and GUI ....... which is why I like PA Vol Ctrl
IMO, it is by far the easiest means of controlling both sound sources and sound sinks where the PC has more than one of either.
