I just had a very similar situation on Windows 10 , OBS 28.0.3. Before a church service everything working on OBS except there is noting in the Audio Mixer window. No meters for any of the sound devices for any of the scenes. Rebooted computer, re-started Windows. Checked sound was coming in from X32 Behringer to Windows using windows settings for sound. It was. Created new sound devices in scenes to see if they would appear in the Audio window. They did not. (e.g. laptop mic). Eventually I changed a scene and the sound source from one of the cameras (which we don't actively use) appeared in the Audio window. Changed scene again, and suddenly our "Church" sound source which is the Berhinger X32 coming via USB input, appeared in the main scene, and then all other scenes. So very strange, and rather stressful.
I enclose the log file in case anyone technical is able to divine what is going on !
Best wishes, Pete