You can access all the channels on your soundcard's mixer with wmmixer. Most common channels are identified with an appropriate icon. Controls include a stereo (mono where appropriate) volume control and a recording source toggle button. Press the "<" and ">" arrow buttons to select a channel. The icon identifies the current channel (a volume symbol with a question mark represents an unrecognised channel). Click and/or drag the volume display to set channel volume. There is a also a button to set recording sources.