Doesnit beep when you turn it on? Whenever I buy a computer from a thrift store I plug it in and see if it beeps. Also if the num lock indicator on the keyboard flashes briefly. If you don’t have bios output to the display then it is not any use to try to figure out which boot device you should use. It has to get far enough to boot.
No beep, no vid at any point, but most everything else seems to be running. Num Lock does come on and stays on if not turned off, at power up.
I’m not sure there even is an internal speaker - if there is, it’s well hidden. Could be some tiny thing on MB, I suppose, but this machine has 7.1 Dolby built into the MB.