I'm aware this question has been asked a few times, but none of the answers did help during my 18.04 time. My mainboard is an Asus Prime H310M-AR2.0 with 3 jacks in the back for 5.1 sound. mainboad description: "Realtek ALC887 8-Channel High Definition Audio CODEC".
My system is dual boot with Windows 10 and Ubuntu LTS. Until two days ago it was 18.04, where there was stereo onl, too. Now I have upgraded to 20.04. I was told by a journalist the upgrade could possibly help, but it did not.
My speakers is a Logitech 5.1 surround system with 6 speakers and 3 jacks. Running Windows 10 I can hear all 6 speakers separately, so the hardware is definitely OK.
But Ubuntu always shows stereo and I can test front left and front right only. "PulseAudio Volume Control" only allows "Line stereo (connected)" and "Headphone (not connected)". A connected headphone works fine with stereo, b.t.w.
Is there a way to reset Ubuntu's sound system completely so that it is newly recognized? Or is such a configuration simply too "old"?