Buy a cheap karaoke-mixer like this.

The audio out of the PC goes to that mixer.
The audio-out of the mixer goes to the amplifier.
If you have a (S)video out then you can put it also to the mixer.
The video-output of the mixer can go to the TV.
But if you don't use the video out, forget this.
Normally it is used between a DVD-player and a TV.
You have 2 channel microphones with volume.
And also echo and tone.
It works fine with me.