The CSND service talks to the DSP using the DSP FIFO to negotiate what CSND channels are allocated to the DSP, and this seems to be channels 0-7 (usually). The rest are dedicated to CSND services.
CSND service "csnd:SND"
A maximum of 4 service sessions can be opened at the same time for this service.
For cache operations addresses must be in 0x14000000-0x1C000000 range. Starting with 8.0.0-18 addresses in the range 0x30000000-0x40000000 are also allowed.
This service is normally used for playing CWAVs. The audio data must be located in the 0x14000000 virtual memory region, since CSND requires the physical address for playing the audio.