unfortunately it's not the case with the new Mac Mini M1. Apple does not allow sound control neither via the Magic keyboard neither via the top right volume setting control. However this works absolutely fine with a windows machine; therefore i was looking for the buttons directly on the monitor to adjust the sound volume.

Well normally if connected to you computer properly , this option "U34G2G1intel (R) Display Audio " should be available in your sound output possibilities. You have just to select it instead of your computer normal audio output and then you will be able to control the sound