Firstly, well to the Apple Eco-System
To the issue at hand
What you’re seeing is actually a long-standing macOS behaviour, not something specific to your Mac mini or the newer macOS versions.
When audio is sent over HDMI or DisplayPort, macOS often treats it as a digital output with fixed volume, which causes the volume slider to be greyed out.
Why it worked in Windows
In Windows 11, the OS often applies software volume scaling to HDMI audio devices automatically. That means even if the monitor doesn’t support hardware volume control, Windows adjusts the signal before sending it.
Why macOS disables it
In macOS, Apple typically expects one of these situations when using HDMI audio:
A - External device has its own volume control (monitor, TV, receiver, soundbar).
B - Volume is controlled by external hardware (keyboard attached to monitor, remote, etc.).
C - The output is treated like a digital line-level signal, so macOS locks the volume at full.
So if your monitor speakers have no physical volume control, you end up stuck with max output — exactly the situation you’re seeing.
Caveat, I have never used either of the below softwares so can not vouch if this will work
Install a small utility
The app SoundSource or MonitorControl is the most popular solution.
Download SoundSource or MonitorControl and install it.
They say your volume keys should start controlling monitor audio again.
