If you are using a VGA cable to carry the video signal from your PC to your touch screen you must also use a 3.5mm male to male audio jack cable to carry the audio signal. If you are using a HDMI cable to carry the video signal this will also carry the audio signal as well so there is no need to use an audio jack cable.
Once you have checked this please ensure that your “Surround Sound” setting is set to “Surround”. To do this press the “Menu” button on remote control, then press the right navigation button, and go to the “Sound” sub menu. Change the “Auto Volume” to “ON” if it isn’t already and change the “Surround Sound” to “Surround” (please do not select the “SRS TruSurround XT” option).