The only thing I can think of is to get two splitters and put one on your speaker output and one on your mic in. connect one speaker out to your speakers the other to the splitter on the mic in. You can then plug your mic into the other connection on the splitter. How good the sound will be I dont know.
First thing that came to mind.
1) Go to your windows volume control panel. You can get there by double clicking on the "volume" icon in your taskbar by the clock in the lower corner.
2) Click on Options > Properties > Select Recording > then hit Ok at the bottom.
3) You should see Microphone selected at the bottom in this panel. Uncheck your microphone and put a check in the panel that says "What U Hear" or "Stereo Mix".
4) Now all you have to do is when you play your sound file.. hold down your Press to talk Button.. or if you have voice activation .. before you play the file go into preferences and lower your voice activation all the way down.
To change it back to your mic.. just go back to the Recording Control panel and put a check in microphone instead of the Mix or What U hear panel.