Cheap and easy way to downmix 5.1 to PCM?

Hello,

I have the comcast cable box with the HDMI.

I have a 2 channel stereo that I use with my TV.

I need to run the audio digitally through my DVDO processor to add lip-sync correction to the cable box, but since the cable box can't downmix to 2 channel pcm, I need some other way to do that.

I would think it would be a pretty easy digital conversion? Any "black box" I can buy to do this?

thanks
Todd