As far as I can see both my clips have the true 5.1 as Stream 1
Correct.
The AC3 clip - Stream 2 is marked MPEG - but I don't hear any audio when switched to it in VLC
In my tests, VLC does play with sound, using stream 2, but since it is stereo only, it plays the L and R channels with sound, all other channels are silent. Now (after my fix today) MC does the same.
The AAC clip - Stream 2 is marked as 5.1 - but again - I don't hear any audio when switched to in VLC
With stream 2, it is total silence, in VLC and in MC (with my fix today). So the clip probably does not really have audio in the second stream, although it says it has a second stream.
So for both clips JR is playing Stream 1
Yes. In a few days when you get a new build with my fix today, you will be able to switch to other available streams.
The question is if sheppy99's clip has Stereo on Track 1 and 5.1 on Track 2 - how does he select which track to hear - don't think JR has a 'track' choice like VLC - probably needs a sample from sheppy99 to resolve.
I fixed that. Wait for a new build to come out in a few days.
Thanks for confirming that there are two advertised audio streams in your test file, which I also confirmed with VLC.