There are numerous MAJOR problems with your request.
1. There are NO free public decoders for Atmos, DTSX, or A3D.
2. Commercial decoders are available, but cost thousands.
3. FLAC is limited to 8ch, but those formats above are 10+ch. (WV & WAV can store ~32ch btw)
4. A channel based formal such as FLAC cannot store object metadata
There are some Atmos & DTSX samples here, but they obviously require you to already have a working Atmos, DTSX decoder/AVR (which you don't)
Wav or Wavpack (or ALAC or whatever) would be fine.
Yeah... Not taking a chance on a Dolby product unheard and that then requires purchasing and setting up over 60 speakers! Not with their past reputation anyway. They truly have implemented some great ideas in the past but also the polar opposite. Goes without saying that I'd want to go all or nothing on the speaker array as spec'd. No interest in downmixing the formats to standard 5.1 - that's not going to tell me anything.
Some synth band with a few polite wiggles and warbles isn't going to do it. Roger's tired karaoke Wall performance sure isn't! Honestly, the only "marketing" I see is selling these scam 'shitbar' speakers with Atmos as a buzzword! I'm about as receptive to that as one of those $1000 HDMI cable scams.
Somebody needs to sell this! Point me to some actual discrete 3D mix that will actually make me hungry to set up more speakers to hear it in 3D perspective with discrete madness. Something refined with spacial tricks could be of interest but a demo should include something gratuitous that genuinely can't be done in 5.1 to grab one's attention.
Someone mentioned music club interest. I've thought about freaking out an audience before with things like changing the room dimensions aurally. Put them in a closet one minute and then dangle them over the grand canyon the next. In addition to discrete motion. Could be a lot of fun! But back to the reality of the average club with half the speakers and equipment blown up, the other half wired wrong, and a "sound guy" that listens to mp3's on a phone and doesn't even know how to hook anything up, let alone run it...
Lots of truly great inspiring talk here but someone needs to walk the walk!