I assume you are using XP. Try these steps.
1. Open 'Sound and Audio devices' in Control Panel.
2. Select 'Advanced...' button near bottom.
3. Select 5.1 system
4. Start playing a sound.
5. Set the volume control of the subwoofer to the middle position.
6. Open volume control and adjust Master volume to a comfortable level (i.e. until there is no hum).
Also make sure you haven't muted any sound device except microphone. I think your problem is you have set it to the maximum level. So decrease it.
7. Mute the sound and check.
Good luck! Let us know the result.
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