Ok - PCM can be streamed to each(or all) of the huc6280 6 sound channels when in those channels are set to DDA mode. The frequency is how fast you can write to the port (higher than 44kHz is easily possible), though it's more realistic to use the internal timer for writing PCM sample effects (up to 7kHz).
The CD base unit adds two things to the PCE - an ADPCM playback controller with it's own 64k of memory and 64k of CD ram to hold game code, etc. You can stream audio into the ADPCM buffer (like many games to for in game voice intermission), but you can also load multiple samples into the ADPCM ram and play them at will. Both Gate and Lords of Thunder use the ADPCM controller for some of the more advanced sound effects with no hit to the CPU resource/cycles (plus is save memory by having the samples in ADPCM memory instead of CD RAM). You can play each sample at a different kHz rate (up to 32kHz - mono), but only one sample at a time. The ADPCM controller used in the CD unit clips at 12bit.
The ADPCM unit is independent of the CD load controller. You can play ADPCM samples while loading from the CD unit or playing CD audio.
Hope that helps.
