I had to install Lubuntu ver. 16.04 (and not, say, 18.04) as my old machine wouldn't tolerate newer versions. Everything is working (and the machine seems to have had a burst of life) apart the TV audio. I have linked my machine to my TV through a HDMI cable that only carries the video signal (it is a DVI to HDMI cable, as my machine doesn't have a HDMI port); so, for the audio signal, I am using a separate audio cable.
About the sound:
- it works if I'm using an headset (both from the USB port and from the audio sockets in the front of my desktop PC)
- it actually works through the TV speakers the moment I remove the video cable from the HDMI TV socket and it stops working again if I plug the video cable back in.
so I am conjecturing that the HDMI signal gets in the way even if no audio is associated with it.
Furthermore, Pulseaudio seems to be working well as it is shown in the enclosed screenshot. I tried all the configuration choices in the Pulseaudio menu with no luck.
here's also the gnome alsa mixer GUI.
Final note: I have a dual boot system with Windows 7 and Lubuntu 16.04; and in Windows the TV audio (with the cable connections I have described above) works.