I have developed the same problem but now different - noted the sound volume issue but now it is splitting the ATC and other sounds between headphones and speakers, so if I plug in headphones I get engine, environment aircraft sounds through the headphones but ATC comes up on the speakers. Not sure where to resolve this as Win10 offers about 3 to 4 sound controllers (NVIDIA, RealTek and the Speaker system plugged in). Seems to confuse stereo sound with the hardware inputs/outputs etc.
On that issue ATC, I am about to ditch the use of ATC anyway, always dubious in FSX now stupid and worse P3D (very slow to respond to flight paths weird heading and paths to various points etc but completely erratic and over the top with frequency changes every 30 secs sometimes) how about recent vector onto a visual final that kept you at 10,000 ft at 5nm from final? Might work out in an F35 but in an old 4 engine piston ah nuh not without a 4000 fpm ROD to get to the threshold. But I digress.