Rockbox hat damals (2005!!!) den code von ffmpeg genommen:
https://git.rockbox.org/cgit/rockbox.git/tree/lib/rbcodec/codecs/libffmpegFLAC
Ich kenne mich nicht so gut mit codecs aus, aber eventuell ist die Datei README.rockbox interessant:
It was also changed to provide output as 32-bit integers (with the data left-shifted to 28 bit accuracy). The original flac.c provided 16-bit output. In order to minimise memory usage, a small number of hard-coded limits are present in decoder.h - mainly limiting the supported blocksize (number of samples in a frame) to 4608. This is the default value used by the reference FLAC encoder at all standard compression settings.