well, logic goes that since monitor works (and yes, it gets sound through display port cable) and normal headphones connected to motherboard work, the problem lies in the USB headset.
If the headset has it's own volume slider, that is the most likely culprit (I have old headphones whose volume slider at times "mutes" other side in certain spots) since it either has random stuff between the connectors or just oxidized connection or.. something like that.
Fixing said thing would require taking it all apart which would most likely break it (they were never meant to be self serviceable) but.. using one of the de-oxidizing sprays for cleaning contacts might or might not work, just let it dry for a day or two after applying it and using the slider to get it to spread where it should to avoid problems.
If your headset doesn't have volume settings, then it might be breaking up internal cable and/or connection which causes extra impedance and thus.. lowers the volume.
Edit: Googled CHT909BT and.. it's a wireless stereo speaker system, making all this talk about headsets feel.. odd.
Yes, it supports (should, according to google) bluetooth and aux in (3.5mm jack) and since you are using bluetooth, it might be that the bluetooth adapter in computer could be a problem.
If the volume problems wont' exist when you pair the speakers with your phone, then the problem is not in the speakers.
If problems stay even with a phone and they are gone if you use 3.5mm to 3.5mm cable between phone and speakers or computer and speakers then the problem is the bluetooth receiver in speakers.