We have a flat screen TV with HDMI inputs and outputs, a Roku, a WII, and a PS3.  I am not a big TV guy and I don't care about surround sound.  In fact I got this flat screen used last year -- up until that point we were using a tube TV, if that's any indication of how technically uneducated I am when it comes to home theater stuff.

My problem is that some of the content (coming into the Roku from a Plex server) only has a surround track.  So we get little/quiet/no audio.

I do have a set of powered bookshelf speakers and a powered sub -- this is perfect for us and I don't want to upgrade to some fancy home theater system.

Wondering the best way to concert all the audio signals from the TV into stereo, be it 5.1, 7.1, etc.  

I have seen a few converters online in the $100-200 range but have no idea what I'm looking for.

Suggestions welcome, thanks.

Send all signals to your TV first, then send the TV's audio output to your stereo receiver/amplifier.  Set the TV's menu  to provide 2 channel audio output.  

Audio output might be RCA plugs or optical (TOSLINK) or,...

What gear do you have?

Is the goal to have surround sound or not?  If you don't want surround and just want to use the TV's internal speakers, just make sure all of your devices are set to PCM, 2CH or Stereo (each device labels it a little different.)  Using the Roku as an example, go to settings, system, audio and make sure it's set to "PCM (Stereo)".  It sounds like its set to DD, DD+, DTS, etc right now but only hooked up to 2 channels (left and right) therefore you are missing the sounds produced by the center and two surround channels.

Do the same thing for the other sources such as PS3 and the Wii.  Make sure your TV is set for Stereo also under it's Audio settings menu.

I don't want surround sound.  But I'd like to be able to hear all of the channels (5.1, 7.1, etc) via two stereo speakers and a small sub.

The TV is a 40" Samsung, many years old, friend gave it to me and I repaired it, now the wife is in love with it.

The Roku is set to "Stereo" and "HDMI Stereo (PCM)"

I copied our DVD and Bluray collection over to my media server so my wife can watch them via Plex on her ipad.  The issue is, several of the Blurays *only* have surround tracks, and when watching them on the Roku, yep, we don't get the center and surround channels.  So we are missing out on a lot of audio, or the audio is very very quiet.  The ipad plays everything fine, and Plex is essentially set up the same on both devices, as far as playback and audio are concerned.  When using the TV's built-in speakers, the effect is the same as when piping the audio out via RCA to our 2.1 setup -- missing audio.

I was wondering if there was an external adapter I would have to use to encode (decode?) surround audio tracks into stereo.

Thanks.

A home theater receiver will do this. Everything hooks to the AVR (audio video receiver) via HDMI cables.  The AVR decodes and plays the audio and it sends the video signal to the TV. Your TV just becomes a monitor. When you change sources the AVR acts like a switch board and does the changes. You no longer change Video 1,2,3 on your TV as you move from different sources (cable box, dvd player, or gaming system).

This place sells factory refurbished AVR's at a pretty good price. You can get a fairly good receiver for the same amount of money you would spend in a retail store.
http://www.accessories4less.com/make-a-store/category/avreceiver/home-audio/receivers-amps/home-theater-receivers/1.html