When I plug in headphones (I’ve tried several) ATC goes to the headphones, but the sound continues to come from the speakers. The two outputs in the Sound settings are set to ‘System’sDefault Device’.
The is Windows 11, by the way, which is possibly the culprit. I’ve tried playing with both the MSFS settings and the Win 11 sound settings but just can’t stop the sound coming from the speakers with headphones plugged in. To put it another way, I can’t get the plane sounds to come through the headphones at all. Can anyone please help?
Right click you speaker bottom right of desktop and select “Open Volume Mixer”
Check the “Output Device” is set correctly.
Find below MSFS (need to be running), then click on the title, it will open option for OutPut / Input. Make sure they are set correct
Failing all that try “Reset” sound settings on same page.
I found the answer… In the Win 11 Sound settings, right at the top, there’s ‘Choose where to play sound’ and this was set to ‘Speakers’. Unsetting this seems to have fixed this.
There used to be a sound setting, in Windows, that I set years ago and never needed to change, that will use your speakers until you plug in a set of headphones, but if you’ve found a solution, all’s good. 
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I haven’t fixed the problem properly - it reappeared. Grrr.
It was hardware - the headphones jack has to be pushed in only half way otherwise you get the sound out of both speakers and headphones. A feature? Probably not.