Go into the control panel. Under sounds, under recording devices, disable internal mic. Set other input as default. There might be more than one listed, find the one that is tied to the scanner.
Right click on the speaker icon. Open volume control. Select Options -> Properties. Click the Recoding option (Playback is typically selected). Click OK. That should bring up your Recording Mixer. You'll not only want to select the desired recording source, but also set the level.