More debugging… I was out of luck because nxplayer just blocks when I tried to play some audio, so I modified nxplayer_main.c to just read the argument from argv[1] and play it. This way I could start up nxplayer in background and run dmesg to see what is happening:
NuttShell (NSH) NuttX-12.5.1
nsh> mount -t tmpfs /tmp
nsh> rz
**B0800000000022d
nsh> ls /tmp
/tmp:
yes.wav
nsh> dmesg
board_cs4344_initialize: minor 0
stm32_i2sbus_initialize: port: 2
i2s_dump_regs: I2S2: After initialization
i2s_dump_regs: CR1:0000 CR2:0700 SR:0002 DR:0000
i2s_dump_regs: I2SCFGR:0000 I2SPR:0002
i2s_dump_regs: PLLI2SCFGR:44013000
cs4344_reset: WARNING: MCLK could not be set on lower half
i2s_mckdivider: Entry
i2s_dump_regs: I2S2: After i2s_mckdivider
i2s_dump_regs: CR1:0000 CR2:0700 SR:0002 DR:0000
i2s_dump_regs: I2SCFGR:0e00 I2SPR:020d
i2s_dump_regs: PLLI2SCFGR:20003540
cs4344_setbitrate: sample rate=16000 nchannels=1 bpsamp=16
audio_register: Registering /dev/audio/pcm0
nsh> nxplayer /tmp/yes.wav &
nxplayer [10:100]
nsh> NxPlayer version 1.05
h for commands, q to exit
nsh> dmesg
0x46464952
pcm_dump: Chunk Size: 4292
pcm_dump: Format: 0x45564157
pcm_dump: Format Chunk:
pcm_dump: Chunk ID: 0x20746d66
pcm_dump: Chunk Size: 16
pcm_dump: Audio Format: 0x0001
pcm_dump: Num. Channels: 1
pcm_dump: Sample Rate: 8000
pcm_dump: Byte Rate: 16000
pcm_dump: Block Align: 2
pcm_dump: Bits Per Sample: 16
pcm_dump: Data Chunk:
pcm_dump: Chunk ID: 0x61746164
pcm_dump: Chunk Size: 4256
cs4344_configure: ac_type: 2
cs4344_configure: AUDIO_TYPE_OUTPUT:
cs4344_configure: Number of channels: 1
cs4344_configure: Sample rate: 8000
cs4344_configure: Sample width: 16
cs4344_configure: ERROR: Unsupported combination of sample rate anddata width
pcm_enqueuebuffer: ERROR: Failed to set PCM configuration: -22
nxplayer_enqueuebuffer: ERROR: AUDIOIOC_ENQUEUEBUFFER ioctl failed: 22
nxplayer_playthread: 0 buffers queued, running=1 streaming=0
nxplayer_playthread: Playing...
nsh>