You should be able to go into the settings of the internal soundcard and change it to a line input. right click on the speaker icon in the bottom right corner, click recording devices, right click on the correct input and have a look through the settings. It may even just be a case of turning down the microphone boost (most are usually set with a default of +20).
The other option, if you're not using channels 1 and 3, is to run the record out via an rca cable into either channel 1 or 3, put the output source back to 1/3, keep the volume fader of that channel down (use the gain knob to control volume) and then try selecting 'USB Audio device' on the webcam screen.
Bookmarks