There's an existing RockBox port, which is great! But it does consume battery power half again faster than the stock Hiby player. Also, and I can't for the life of me tell if this is a defective unit, the Rockbox port is badly coded, or the underlying firmware is utterly stupid, BUT it continually corrupts my music.
Using MD5sum, I have now confirmed that a file I copy over USB, reaches its destination with all the bits in the same order. But even as fast as six or seven songs later (might equate to one, or three power down/up cycles) that particular file will come up in the playlist, and it "glitches" for a second. Files that are in the middle of giant gigabyte-sized thousand-song copies are often *much* worse off, having half a dozen, sometimes full-second-long sections of unrecognizable, ear-piercing static. File size is never different, but md5sum, again, shows it's no longer the same file as what I copied. Replacing it fixes that problem for a while.
I've tried this with two separate (different brands/vendors) of 128GB SD card, and the player actually destroyed the first one. While copying a 4gb directory, Windows became under the assumption that I had removed the SD card from the player (I hadn't even so much as bumped the cable!) the resulting filesystem corruption (I hadn't RockBox'd the player yet, and telling it to format the card, formats it in fat32) caused the card to revert to an unresolvable read-only status. I mean, I have now roughly 4GB of music I like on that card, and it doesn't auto-corrupt the files any further, but thirty dollars is a bit much for a burned-cd of fanmade game music.
I'm going to experiment with 4GB SDcards. Maybe the USB/sdcard wiring is specifically what's sub-par, and easy-going ten-year-old cards will be more resilient. If the file isn't corrupted, the music is fine, and even the stock Hiby interface is fine, though RockBox is clearly better.