Wallace wrote:
My headphones are connected to a speaker system that's plugged into the monitor's headphone jack. The speaker system has it's own volume control module with a headphone jack on it.
This may sound strange, but it works and there's no need to disable anything. Except when I need to use my headset with a microphone. I'm also able to use the subwoofer that's part of the speaker system, for killer bass.

I see. Some HDMI video cards have an onboard sound chip and you can end up with two sound chips, one on the video card and one being the motherboard's (or sound card's) onboard sound chip. You might be able to configure your HDMI port as a pass through device and use the motherboard's sound chip which might allow you to connect your mic but I don't see how you gain much in the way of sound quality by doing this.
You're probably better off just disabling the HDMI sound and connecting your speakers to the motherboard's (or sound card's) audio jack. That way, you can use your mic too. The way you have it connected now, there is a digital to analog conversion occurring when you connect the speakers to the headphone jack on your monitor. You'd have more control and probably get better sound by using the motherboard's (or sound card's) audio out connected directly to your speakers. Just use the HDMI for video.