An external dac wouldn't help if the signal is being picked up by the wire to the headphones.
HDMI works just fine because that is a digital signal, not an analog signal, I assume. Then the wires from where the HDMI signal is decoded and sent to the speakers is all within the monitor itself, I presume, meaning that the wires aren't long enough to pick up the radio interference. I still think that ferrite chokes would do the trick. You might need to use one on either end of the cable. However, a quick and easy way to check if I am right would be to wrap the cable in aluminum foil. That should shield it. If that works, then you know what the problem is and we can work on proper shielding. If it doesn't work, then we are back to square one.
A cheaper easier way to test it without having to bother with aluminum foil is to coil up the cable and then put it under a metal pot. If that doesn't help, then try the aluminum foil. If either works, then we need to work on shielding the cable.
I think someone may have already suggested it but ferrite chokes would work, they are pretty cheap. it's just a little ring that snaps on to the cable of your headphones, or whatever the hell seems to be picking it up, maybe the plug for the pc? idk. they usually come in packs of a few so you could slap them on everything lol
Sort of trolling and sort of serious. Brass mesh all the walls and ceiling and earth it. Basically make the room a Faraday's Cage. That should stop all interference completely but is also collossol overkill and potentially unsightly and expensive.
To work effectively the meshes all have to be connected and optimally have no spaces on the seams, overlapping would be perfect and the eathr it into the ground.
This would drive me crazy and I would personally go to these lengths. But stuff like this drives me literally crazy, in bad moods and grumpy because someone else is messing with my stuff and there is little I could do. So I go a bit loopy.
It wouldn't kill it assuming that the access point and the devices are both within the cage. I have often thought about building a house and adding a mesh to the insulation to make the entire house a giant faraday cage for the hell of it. Of course, that would mean that cell phones wouldn't work, so it isn't too practical for me, but still and interesting idea.
I´m trying to figure out what radio station it is and call them to troll the shit out of them. I don´t think that if I wrote them a friendly email they would change their frequency.
Do not be surprised if I buy like a circus lion cage and put my desk in it. This is driving me nuts, specially from around 8 to 11. That program is the worst.