Originally Posted by bowmah
I contacted one of the developer (not sure which app anymore) but at that time, yes, Apple does not allow 3rd party apps to output 5.1, only Stereo. Apparently, Airplay from a 3rd party app is not identical to Airplay from a native iOS app. Hence, you get 5.1 when you buy / rent iTune movies but not with XBMC or AirVideo. This was some time ago but I am pretty sure it's still the same now.
Just to clarify, if you rip your own movies and convert them with Handbrake into mp4 files, you can keep the 5.1 audio when you play them directly on the ATV, and I think also if you play them on an iOS device (using the built-in Video player) and then stream them via AirPlay to the ATV. So the limitation (apparently imposed by Apple) seems to be with the video player that developers can use within their iOS apps.
My personal take on all of this...I would love nothing more than for Apple to open up the ATV further for 3rd party apps so that we could get Plex on their directly, complete with 24fps and 5.1 audio (the former may not be possible due to hardware limitations, but perhaps that's not the case), but the current limitations (stereo only, no 24fps, and the need to use Plex/AirVideo on an iOS handheld and then stream it over to the ATV) is still an acceptable way to play stuff on TVs in rooms that lack 5.1 audio and/or where 24fps video will not be a big deal. In my case, I like having the one centralized Plex media server running on a single computer in my home and keeping track of all of the TV show and movie metadata. That computer happens to be connected to my largish LCD TV in my bedroom (just the built-in speakers). For my living room / theater, I have 5.1 audio and a front projector. For that room I have a separate HTPC running the Plex client app and playing back this content, complete with 5.1 audio and 24fps video. For a guest room, or even a master bedroom, I don't think the iPhone Plex to ATV approach is unacceptable.
So, I think the key is to have a single decent computer running Plex media server and have that connected to your most important room's display. FWIW, my main computer/server is an obscenely large HP tower (Intel i7 CPU) which actually runs fairly quiet. But I recently splurged and bought something I don't really need: a base-model 2012 Mac Mini (from Best Buy as an open-box deal). I'm thinking about replacing that HP tower with the Mini. It's less powerful, but sexier looking and will draw less power. Since I run Windows 7 Media Center on my tower (connected to a SiliconDust HDHomeRun Prime 3-tuner cablecard tuner) it stays on 24/7. The move down from that tower's i7 CPU to the Mini's i5 (and a laptop-variant of the i5 at that) could hurt on-the-fly transcoding performance but, to be honest, I don't really do a lot of that anyway, since (as I mentioned) my living room has its own HTPC anyway (a low-power Acer Revo 1600 Atom/ION nettop). If I was really needing to process multiple streams of on-the-fly transcoding to multiple iOS devices in the house simultaneously, I'd probably stick with the beefier i7 tower PC.