With a Mac mini m2 and lg display how do I control the monitor volume with a Mac keyboard

You don't via HDMI. See the LG controls (e.g, joystick) for audio adjustments using its internal speakers, or just connect Bluetooth headphones or sound bar to have the ability to use the keyboard shortcuts for volume adjustment.

My LG 32UN880-B has a recessed joystick on the bottom of the display which isn't much fun for adjustments of any kind, and I leave the 5 watt LG speakers off and use Bluetooth with my Sony WH-1000M4 headphones or the M2 Mini Pro internal speakers in a pinch.

Use HDMI-HDMI for video and connect the LG’s USB C to the mini’s USB A port using an A-C cable/adapter, should fix.

Typically it's this:

F10 - Mute/Unmute

F11 - Volume Down

F12 - Volume Up

Exactly what model Lg monitor are you using?

How is it connected to the Mac?

Sometimes a monitor will require a USB connection to the Mac in addition to its video link in order to control some functions.

The LG monitor type: 27UP85NP

I can control the sound through the monitor boxes only when using the "joystick" at the bottom of the monitor.

Subject of my search: using the key board buttons F10 F11 F12

Connection to the Mac mini 2: UPSTREAM SS

Tkm for helping me.