How to adjust volume on a Multi-Output Device?

jjjefff wrote:

When I set the sound output to a Multi-Output Device, the volume control no longer works....

that is expected behaviour.

Is it possible to set the volume keys to adjust the Macbook Speakers while the output is set to a Multi-Output Device?

sorry, no. you cannot control the volume in multi-out mode. you must set the volume to a comfortable level before choosing multi-out.

Third party software might help. One is free but has not been updated in over four years. Another is pricy. Some users have attempted to write their own apps and made these available online, but other users of these have noted various bugs (e.g. has to be reinitialized after sleep mode). Adjusting levels of the individual devices in System Settings / Sound (Sequoia; different in others?), while not as convenient, is the surest method.