Just to add my findings on playing mkv files in case it helps anyone. I appreciate some of this has been said already. I should point out I have just the basic HD-Fox box not the HDR model but I don't think that matters.
I think the box has problems with the headers of some mkv files. If you open the files with mkvinfo, which comes with mkvtoolnix, you can see the information in the headers in more detail. All the files I have that won't play are like this:
Files missing this extra data work fine:
I'm not sure if the problem is this data itself, or just that files with this data present have some other issue that stops the box playing them. Originally I was sorting this by remuxing the problem files with an older version of mkvmerge (v2.6.0). mkclean mentioned above works fine (thanks Rich_A) and is a bit faster. Of course if you're remuxing you could just make .ts or .mp4 files as has also been mentioned.
I think having header compression in the files is also a problem, remuxing or mkclean should fix this. If you're using mkvmerge with some versions you can/should set the header compression to "none" for each track in the file.
The audio format in the files matters as well, out of the ones I've tried:
AC3 always works fine (including 5.1)
AAC works at least some of the time
DTS will not play (you get an error message and no sound)
If you have files with the wrong audio format you can convert to AC3 with this program and retain 5.1 sound if you want to, as well. For files with multiple audio tracks (e.g. an extra commentary track) the box just seems to pick one randomly. You can remove tracks with mkvmerge.
Internal mkv subtitles are ignored. srt subtitles in the same folder with the same name do work OK though they were a bit distorted when I tried (think this is due to the aspect ratio problem below).
The box seems to completely ignore the aspect ratio of the video and just fills the screen. For cinema-style widescreen that should be letterboxed, you can get around this by changing TV type to 4:3 in the options menu and choosing the letterbox mode. I read that in another thread somewhere on this forum, so many many thanks to whoever pointed this out originally.
FF/rewind does work with mkv but it's not very smooth. Skipping back/forwards with the left/right arrows works fine.
I think the box has problems with the headers of some mkv files. If you open the files with mkvinfo, which comes with mkvtoolnix, you can see the information in the headers in more detail. All the files I have that won't play are like this:
Files missing this extra data work fine:
I'm not sure if the problem is this data itself, or just that files with this data present have some other issue that stops the box playing them. Originally I was sorting this by remuxing the problem files with an older version of mkvmerge (v2.6.0). mkclean mentioned above works fine (thanks Rich_A) and is a bit faster. Of course if you're remuxing you could just make .ts or .mp4 files as has also been mentioned.
I think having header compression in the files is also a problem, remuxing or mkclean should fix this. If you're using mkvmerge with some versions you can/should set the header compression to "none" for each track in the file.
The audio format in the files matters as well, out of the ones I've tried:
AC3 always works fine (including 5.1)
AAC works at least some of the time
DTS will not play (you get an error message and no sound)
If you have files with the wrong audio format you can convert to AC3 with this program and retain 5.1 sound if you want to, as well. For files with multiple audio tracks (e.g. an extra commentary track) the box just seems to pick one randomly. You can remove tracks with mkvmerge.
Internal mkv subtitles are ignored. srt subtitles in the same folder with the same name do work OK though they were a bit distorted when I tried (think this is due to the aspect ratio problem below).
The box seems to completely ignore the aspect ratio of the video and just fills the screen. For cinema-style widescreen that should be letterboxed, you can get around this by changing TV type to 4:3 in the options menu and choosing the letterbox mode. I read that in another thread somewhere on this forum, so many many thanks to whoever pointed this out originally.
FF/rewind does work with mkv but it's not very smooth. Skipping back/forwards with the left/right arrows works fine.