[AdamBrunt]

How do the Netflix and Amazon addons choose the default audio stream ??

In my Kodi setup - for some reason - the audio settings have the number of channels set to 2.0. But the addons still default to the 6 channel DD setting and I hear no sound (or occasionally very weird effect where you hear the background music and very faint voice channel). If I change the stream to the DD 2 channel setting then I get sound. I have tried changing the audio setting (in Kodi) to 5.1 (and the Pi is connected via HDMI to a 5.1 amp) but still hear no sound when the addons used the 6 channel DD stream.

I have tried changing the audio stream, in Netflix, to the 2 channel DD and clicking on the make this the default setting but when I switch to Amazon the audio stream is back to the 6 channel option as (presumably) it doesn't have a 2 channel DD option (just a 2 channel AAC one).

Does that make sense to anyone ? Is it possible to set it up so that it never picks the 6 channel option ? Or is my setup wrong - and 6 channel sound sould be possible ?

 Your setup sounds wrong. Many video files only include a 6 channel audio track, and many users have only stereo speakers - with the correct settings kodi will decode the six channel audio and downmix it to stereo.
You don't need a dedicated stereo track.

I'd start by choosing "reset all options to default" in the setting/system/audio. That should be configured for stereo output. Does that work for you?

I suspect you either don't have number of channels set to "2.0" or you have passthrough enabled currently. And your 5.1 amp isn't configured properly to handle 5.1.  

 Thanks for the help popcornmix.

The number of channels is definitely set to "2.0" (as per the KODI Raspberry Pi recommendations I found somewhere else) and I have passed through enabled as well. The sound comes out via an HDMI lead to an HDMI switch (which then then sends the video signal via HDMI to the TV and the audio via a digital optical cable to my Yamaha VS-10 5.1 system which is also DTS capable)

Interestingly ... the manual says the following 'programmes/effects' should be available:

1. VIRTUAL SURROUND (input source: Dolby Digital, Dolby Surround or DTS)
   
2. DOLBY DIGITAL (input source: Dolby Digital) / DOLBY PRO LOGIC (input source: Dolby Surround) / DTS (input source: DTS)
   
3. DIGITAL MOVIE THEATER (input source: Dolby Digital) / 70mm MOVIE THEATER (input source: Dolby Surround) / DTS MOVIE THEATER (input source: DTS)
   
4. GAME
   
5. HALL
   
6. CONCERT/SPORTS
   

But when I loop round the effects I only see the bolded items suggesting that that amp doesn't think it is getting (or can output ??) a Dolby Digital source