I made a discovery... I was playing around with different file types and found out a few things...
- it still doesn't play FLAC files, the system doesn't see them at all
- it will play wav files but no more than 2 channel. Tries to play a 6 channel WAV but errors out. WAV is a good solution to those ripping CDs to FLAC. Rip them to WAV instead. It'll be a huge file but 128gb sticks are $30 these days. This will give you uncompressed audio off the USB
- While the system has the DTS codec and can play it off a DVD, it can't seem to do it when DTS audio is embedded in a MKV. Maybe it's because I didn't transcode the audio to DTS, I left it at DTS 96/24. I'll try it again tonight after a transcode.
- Audio in a MKV that works is AAC, AC3, MP3, Vorbis, and oddly enough FLAC.
- Like a DVD, playing video files when not in park is audio only. You can still browse your folder while driving.
Sound quality isn't great, probably because I only encoded at 320kbps when the original DTS file is 1536kbps. That's only 53kbps per channel. Once I find a good combination of codec and bitrate that sounds as good as DVD, I can embed my 6 channel FLAC files in MKV files and finally play them through USB instead of burning to a DVD.
You can also use this method to play FLAC files ripped from CD but it seems like extra work, I would just rip to uncompressed WAV like I mentioned above if you don't mind the larger file sizes. Neither format will let you embed tag info though so if you want song names, album art, etc then this isn't for you.
It's a workaround to be able to play FLAC but you technically can if you want to put in the work... lol