Just to confirm for anyone else experiencing the same problems.
I had no sound on the headphone port of my SoundBlaster Zx card after a fresh install of Manjaro KDE.
After launching alsamixer from the terminal selecting the SB card with F6 and using the output select to select headphone the problem was 100% resolved for me.
It would be nice to have these mixer controls in the GUI.
I installed KMix but it didn’t offer any more sliders than the mixer in Manjaro. Why?