I used aplay to play movie output piped from ffmpeg on my kindle. I will publish a script soon to play movies on with sound on your kindles (from local files or internet URLs) in various formats (including x264 for the faster kindles). The K3 is not fast enough to decode x264 in real-time, so it can only play XviD/DivX/MPEG, etc.
Thanks for pointing out this aplay program, or my tools would be stuck with only silent films.
The K4 can use aplay to output to a .WAV file, or to pipe it to another program. I think a sound server running on a host PC would be good for a K4. It could then play sound over wifi to a remote sound system (perhaps even to a wifi-enabled smart TV). The simple way would be to pipe aplay to netcat (nc), then on a host PC from netcat to a player program that can accept raw sound from netcat.
And the K3 has aplay built in too. You can test it with:
To play "s16le" formatted output from ffmpeg, you need to add a "-f cd" to aplay:
ffmpeg ... -|aplay -f cd
I will post ffmpeg with I have scripts that set all the parameters to sync the video and sound reliably for various movie format. Just like sound is delivered to aplay here, the video is delivered to gmplay used in the current "kindle video" thread. These audio and video programs naturally belong together.