I'm sure you could do that with software (ie the windows mixer), but I don't believe the quality would be very good over a PA system. You would be better off going to your local music store and buying a cheap handheld mix for like $25....
Something like this would give you much better sound quality and is very affordable.
DJM-200B
600 Ohm microphone
MAIN FEATURES:
� 600 Ohm microphone
� Low noise - Low distortion
� ON-OFF switch
� XLR connection
� Includes 15 foot detatchable XLR to 1/4" cable
� Great for DJ�s, karaoke, churches, lecturing, or anywhere a quality microphone is needed
� Frequency Response: 50 - 13,000Hz.
� Direction: Cardioid pattern
Price: $24.95