audio_sound_load_apcm() loads the .adp files that Open PS2 Loader uses. A file has a 16-byte header followed by PS-ADPCM blocks of 16 bytes (28 samples each).

Offset	Size	Field
0x00	4	APCM
0x04	1	version, 1
0x05	1	channels, 1 or 2
0x06	1	loop, 1 to play until stopped
0x07	1	reserved
0x08	4	pitch value written by the encoder (little endian)
0x0C	4	sample frames per channel (little endian)
0x10		ADPCM blocks. Stereo: every left block, then every right block.

The header has no sample rate; the library plays these files at 44100 Hz. Each channel's run of blocks ends with one extra block that the encoder flags to repeat itself. The library rewrites the loop flags when it uploads a sound: a sound that plays once ends at the last block of each run, and a looping sound repeats each run from its first block.

audio_sound_load_adpcm() takes the same block data without the header, with the rate, channel count and loop setting passed in a audio_sound_desc_t.