Decode DD+ to DD?

Not Sure what you are asking. But My Samsung Blue Ray player has a decoder in it. I have it setup as a DTS converts to Dolby Digital. I went in through the settings option and it allowed me to do it. Now I can watch any disk in Sonos Surround. Is that what you ment? It should work for Dolby Digital plus too. I would just see if you can return the player if it doesnt do it. But I thought most Samsung Blue Ray players have this option. With Netflix, Hulu and Amazon Prime I dont have the issue because all of their data seems to be streaming in Dolby Digital Surround.

I guess the OP is looking to stream Netflix, not so much view physical media.
As far as I know, there is not really a cheap, small device that could be used for on the fly transcoding like this.

With Netflix, Hulu and Amazon Prime I dont have the issue because all of their data seems to be streaming in Dolby Digital Surround.
I cannot speak for the other services, but Netflix uses DD+. If you are getting it to output DD 5.1 to Sonos, as I said in another thread, I am curious to know what devices you are using to make that happen.

I stream Netflix through my Roku device which feeds directly to the TV....Then my TV has Optical out to my soundbar. It sounds great. My wife just commented on how well she could hear office people behind us.