Does anyone else have this problem?

My HTPC has 5.1 analog out. My AVR has 5.1 analog in. My TV has stereo analog in.

If I set the HTPC audio settings to 5.1, the surround sound is great. But sound through the TV is crap because center channel is lost. Wife not happy. Wife doesn't like to turn on surround sound just to watch TV.

If I set the HTPC audio settings to stereo, TV is fine but obviously surround sound is gone.

The solution would be a little gadget that could downmix the 5.1 to stereo for the TV. But I can't seem to find such a thing.

 Try using an RCA to headphone y adapter from the headphone or green speaker out to the television.  I believe that these are automatically downmixed.

The problem with any passive device like a simple Y adapter is that it causes crosstalk back onto the 5.1 surround. So, yes, in theory if you had enough Y adapters you could downmix the 5.1 to stereo but the 5.1 feeds to the surround sound system also get downmixed. The whole system would end up sounding mono!

I gave up trying to find an off the shelf solution and built my own.

It's an active downmixer that takes 3 stereo inputs from my HTPC (Front L&R, Rear L&R, Sub&Center), outputs 6 RCA to my AV receiver and 1 stereo to my TV. The 6 RCA outputs are merely straight through connections of the inputs.

The TV stereo output is a mix of FrontL+RearL+Sub+Center to Left and FrontR+RearR+Sub+Center to Right.

It has to be an active mixer otherwise passive mixing would leak crosstalk to my AV receiver (i.e. all the surround sound channels would get mixed up together). I built it from various components from RadioShack including a couple of quad op-amp chips (impressed that they still carry enough electronic components to build something useful!)

Now surround sounds works if you turn on the AV receiver and if not you just get downmixed stereo out of the TV. Works great and WAF is much improved! 

Are you sending the video to your TV via HDMI?  If so, here is what I do. If I want just stereo through the TV I change the defalt audio device in my Windows sound settings to my HDMI connection to my TV. If I want 5.1 surround sound I set the defalt to SPIDIF ( I use my Toslink optical connection to my receiver) in your case it would be Speakers. Yeah I actually have to get up and walk the whole 15 feet to the computer when I do this, but it works.