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                                                                         
nsh> ls /tmp                                                                    
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                                                                      
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...