I have had the exact same problem problem with my ST2220T Multi-Touch Monitor. The sound in headphones through the audio jack on the side of the screen is quieter than expected.
I have connected the monitor to the computer using an HDMI cable. The Line Out Source in the Audio Settings menu is selected to be HDMI. I also have connected the ST2220T USB cable between the monitor and the computer.
Does anyone know if there is a solution to this issue? Thank you.