[bobones]

XBMC 13 can transcode AAC to AC3 so your receiver "sees" a Dolby Digital signal so all you need to do is replace the pre-installed XBMC 12. You need to set the speaker config option to 2.0 to get the transcode option. This works well in my experience.

But would setting the speaker config option to 2.0 mean that audio is output as 2-channel stereo, rather than 6-channel/5.1 (thus defeating the point)?

Counterintuitively, no it doesn't. Your receiver gets the full signal and will decode it to give you 5.1 or whatever.

I think there is a fair amount of confusion around these options in Gotham, but the bottom line is if AAC isn't getting output correctly from your non-AAC capable receiver as multichannel PCM then you can make XBMC transcode it to AC3 (Dolby Digital), but the option is only visible when speaker config is set to 2.0 (and passthrough enabled, and AC3 capable receiver selected).