One of the standard troubleshooting guides is https://help.ubuntu.com/community/SoundTroubleshootingProcedure Though being somewhat dated, it’s step 4 contains and provides a ton of information I find useful in troubleshooting (esp. giving details of your hardware that make finding solutions far quicker).
fyi: It was first in my startpage search using “troubleshooting sound ubuntu site:*.ubuntu.com” because I limit myself to official sources before I expand to sources I don’t know; lubuntu.me is also official
I’d firstly look at the output of
sudo lshw -C sound or list-hardware of class=sound to see what driver is recognized/used by the kernel (esp. given you’ve found “Dummy Output” in
Please note: Given the output of step 4 in the guide I linked is so large, provide a link to somewhere it’s located if you’d like us to help with it (eg. use
|pastebinit or equivalent). Personally I’d just try the
sudo lshw -C sound step first (going further only if needed)