Why does my speaker connected with aux cable work with one computer, but not the monitor, nor a second computer?

Ok, so I have two computers, a monitor, a speaker and an aux cable.

When I connect the speaker via aux cable to one of the computers it works without issues.

When I connect the same computer to a monitor via hdmi and then connect the speaker to the monitor's aux port it doesn't work (the sound works for some seconds, then off for some seconds and goes like that).

When I connect the speaker via aux cable to my other computer's aux port it is the same issue as with connecting the speaker to the monitor.

Has anyone experienced something like this and maybe know what could be the issue?

Cheers! :)