Virtual Audio Cable
A newer version of Virtual Audio Cable is available.Or, Learn More About Virtual Audio Cable
Full user review
"hard to use"
seems to be a good idea, but...
At first I don't know how to use it. It seems it is made by and for programmer. Then, as other people already said, trial voice repeated agian and again, very noisy.
I find another similar soft: http://www.virtualaudiostreaming.net/, may give a try and post reviews later.