External speaker control via Macbook
I have a MacBook Pro (13-inch, M1, 2020) running Big Sur 11.4. I have it connected to a HP Z27 monitor via a usb-c to usb-c connection. My external stereo speakers are connected via a regular headphone plug, also into the monitor. I am using an Apple bluetooth Magic Keyboard.
Up until 2 or 3 days ago, I was able to use the volume control keys of the Magic keyboard to control the volume of the external speakers. Suddenly, that no longer works. It has been working this way since I bought my Mac one month ago. When I look in system prefs>sound, it says that the HP Z27 has no output controls. Yes, I can control the speakers manually via their volume knob, but I like controlling from the keyboard.
I don't know what changed? How can I get it back how it was? Thanks.
MacBook Pro 13″, macOS 11.4