Could you please edit your question to include the output of the terminal commands lspci -nnk | grep -A3 Audio, aplay -l, and pactl list short sinks? Please also make sure that the relevant audio channels in alsamixer (terminal) or gnome-alsamixer (GUI) have their volume turned up and aren’t muted. However, if the device doesn't work in Windows either it's likely a hardware or firmware issue and unrelated to Ubuntu.

@DavidFoerster Added the outputs and the current state of my alsamixer...

How do you connect the headphones to the computer? Via a standard phone connector jack? Did you verify that it's the right jack (and not the microphone or rear audio jack if there are more than one)? Do other headphones or speakers work on the same jack? The headphone jack is muted according to the screenshot of Alsamixer.

@DavidFoerster It seems that I can only get my earphones working when I plug them in the same port where my main speakers are plugged in... I have three audio ports at the back of my CU box, and my speaker is plugged in the middle one labeled "LINE OUT"- this is the port where I can my earphones to work too; at the front of my CU there are two ports with symbols of a microphone and a headphone but none of those work with my earphone or the speakers... Maybe the front ports aren't connected internally to the CU, that seems to be the only explanation...!

Could you please enable all muted audio channels in alsamixer/gome-alsamixer including those that may require scrolling to the right? The front audio jack is often on a different audio channel than the "main" audio jack in the back.

@DavidFoerster I've enabled everything, as you can see from the picture of the alsamixer above(I've just edited)... I even tried changing the channel...

Hello!

@Apollonius: There are still muted channels (see the MM symbol below them), most notably the "Headphone" channel. You can unmute them if you select them with the arrow keys and press m, so that the symbol below the volume meter shows 00.

Finally! I unmuted the headphone bar as you just said!!

Thanks for putting up with me and all this nonsense... I feel like such a fool!

Eh… as a terminal application alsamixer is not very intuitive to use if you're used to point-and-click interfaces but it's included in Ubuntu by default. I probably should have recommended gnome-alsamixer instead even though it requires the installation of an additional package.

But it seems really simple now that I know it!

About the Loopback section- do I need to enable it for any reason?

@Apollonius I'm not actually sure what that does. That would be worth a separate question. It's enabled on my system for what it's worth.

Okay man! Again, thank you so much...

It's worth to revisit the ALSA audio channel volume settings when a channel doesn't work in the future. I've have situations where PulseAudio (which acts as a front-end to ALSA) muted a channel on command but wouldn't unmute it later.

Got it!