[Apothis]

That's interestng. I could not get the Fire Tv to transcode from 5.1 AAC to 5.1 AC3. What am I doing wrong?
I have enabled HDMI passthrough and have ac3 capable receiver ticked.

Sill there a no 24Hz which is not ideal (I can tell the difference when watching a movie)
but if I could get AAC to AC3 transcoding to work that's a good step in the right direction.

The fire tv is a lot more convenient running xbmc than the raspberry pi as the menus are so slow on the later in comparison.

i also cannot get 5.1 aac to output 5.1 to my receiver from the fireTV, a PC does it no problem, but iirc theres a 3rd option for AAC on the PC version of xbmc that the android version does not have. its only a few movies in my collection but its bothered me since day 1

ME TOO!!! Quite Alot of my movies are 5.1AAC and I cannot get them to output anything but stereo to my receiver. Sounds like someone has gotten it to trasncode to AC3, perhaps there is something in the setup we are missing. I am running Gotham 13.1
Any help much appreciated!!!!!

To get XBMC to transcode AAC 5.1 to AC3 5.1, in the Audio Setting under System, ensure you have Passthrough enabled, and the number of speakers set to 2.0. (This doesnt affect your ability to pass 5.1 audio on the passthrough, as it essentially does nothing for a passthrough device, but the setting does enable the next option). Now under the passthrough section, where you select the capabilities of your receiver, under the AC3 capable reciever, there will be an option to enable Dolby Digital Transcoding. Enable that, and your AAC 5.1 will be transcoded to AC3 5.1 and passed through to your receiver as such.

For the guy commenting that the PC windows version used to have an option for AAC passthrough. I think its been confirmed that that didnt actually do anything as there are very few consumer devices that actually accept AAC like that. Info from this page, http://forum.xbmc.org/showthread.php?tid=152691&page=2 would seem to confirm that.