About Mixing two signals into one

[Bandroom]

Dear,

I am now implementing an online Karaoke system. In machine A, I retrieve an mp3 file from the Windows 2000 server. After that, I input a human voice in machine A. The problem is that I dun know how to mix two signals into one and stream back to the server. Do u know what method I can use?
Note: Mixing and streaming have to be done in real time.

Thx and rgds,

Benny