In the Playback tab in pavucontrol I can set the output device to use for each application but not for the "System Sounds".
I have 2 active soundcards, one for my speaker and one for my headphones. If both are enabled system sounds are played in my headphones and if I disable the headphones the sound is played in the speakers.
My "speaker" output device is already set as the default/fallback device.
Is there any way to change so that system sounds always play on the output device connected to my speakers?
In Ubuntu, pavucontrol and system settings sound are two GUIs for the same backend config. However, they manage to conflict in many strange ways. You will have to ditch pavucontrol. Just do the same thing in sound settings and it will work - works for me. Set speakers as default, and move players to headphones.