Hi,
If you use LV 5.1. or higher then you have a built in Sound Control Toolkit. This is located in "Functions->Graphics&Sound->Sound". There is a set of VIs which allow you to control sound card input and output.
Also you can find a lot of built in examples on how to use these VIs. Go to menu bar of LV GUI and select "Help->Examples->Adavanced->Sound Control->...". May be you will find there a good ready to use VI which you need.
Good luck.
Oleg Chutko.