For A/V movies, we will probably need more Theory of Operation when we start using pthreads so that one thread can be stuck in the eink update syscall while the other thread is stuck in the sound buffer write library call. And maybe a third thread to run the GUI.
And u8 (unsigned 8-bit) mono audio should be fine for tiny little speakers. The viewer will be distracted by the video anyway. Of course, using quality headphones while watching music videos may be a little dissapointing on both the audio and the video, in which case an eink kindle is clearly the wrong platform.
So yes, u8 audio is quite appropriate for long persistence low framerate eink dithered monochrome video, I think...
And besides, the relatively simple C code public domain cellphone codec I am porting for the gmv mono audio track is only 8-bit.
Of course, 8-bit audio needs an audio compander algorithm (dynamic range compression) to avoid severe distortion on low volume audio passages or clipping (fuzz box) on high volume audio sound effects.