[kbarni]

@muggleMode Kindle doesn't support USB host mode (i.e. you can't connect USB peripherals like keyboard, mouse or audio device).

@bozza111 M4B is a container format, not an encoding format; it can use several encoders, like AAC, MP3, OGG... It needs a decoder library like libfaad (if the format is AAC).

I did some quick tests with faad - the debian bullseye armhf version runs on the Kindle - but it stops after a second of decoding. (while running fine on my laptop)
Anyway, it could be possible to modify sox media player to use faad instead of the sox library:

LD_LIBRARY_PATH=/mnt/us/M4BPlayer /mnt/us/M4BPlayer/faad -f 2 -b 1 -d -w "$1" |  /usr/bin/gst-launch \
     filesrc location=/dev/stdin \
     ! audio/x-raw-int, endianness='(int)'1234, signed='(boolean)'true, \
        width='(int)'16, depth='(int)'16, rate='(int)'48000, channels='(int)'2 \
     ! queue ! mixersink&
     ;;