So I just made this same post on Tom's hardware but I thought it could also be an FSX problem. So I just replaced my motherboard because for some reason the old one died and when I loaded everything in FSX up it was fine, but for some reason now, when I load up my PMDG planes, the button clicking sounds and cockpit environment play through my monitor and then the sub sound of the engines play through my headphones. This is going to be a problem when flying on vatsim and I was planning on that today, but FSX being FSX or my computer being itself decided to produce the daily showstopping problem. I just don't understand. Even today it was actually working fine but when I went to listen to LiveATC I wanted to hear it through the monitor so I unplugged my headphones and reloaded it. Since then it has not been working right. I tried plugging my headphones back in and muting the monitor and i tried disabling all sound devices except for my monitor. Any help is appreciated, thanks.

I had a problem with the sound when I first built my new system. I was originally using the default Windows 10 sound driver and, although not the same problem you have (I was only getting sound from one speaker), I resolved it by installing the Realtek sound driver which came with my motherboard. Worth a try if you're using the default Windows sound driver.

Well, I got it to work but I had to use my other monitor and now it works fine.