I have searched and I could not find an app that can use your microphone and the speaker as PC hardware. However you could download 2 useful apps which can probably do the work you want.
Can use your smartphone as a PC speaker.
WO Mic (http://www.wirelessorange.com/womic/)
Can use your phone as a microphone via bluetooth.
You could theoretically use both at the same time but I haven't actually tried this before. Hope this helps