Microphone voice

I have a problems with getting microphone voice to the speakers. HTPC
recognizes the front mic and recording works fine. I tried to adjust
microphone in realtek hd audio control. There is recording volume and play
volume controls. I tried every compinations but nothing through to the
speakers. I check windows own voice control. There is only on/off adjustments
for the digital out put.

Acer idea 510
Windows vista home premium.
Device is connected to the av-receiver by optical wire.

Purpose is play karaoke dvd and sing with the microphone. I don`t know is
this possible?