Wow, really cool, and very interesting use of vocals in there . . . I wonder how much memory those samples took up. ;)
Oh, and on KungFuFurby's comment: good quality samples aren't related to CPU overhead (main CPU that is), since the Z80 is normally used for PCM playback (and often sound chip handling too). It's really a matter of good programming along with ROM space for good quality samples, that and well optimized 8-bit PCM samples in general. (PCM is limited to 26 kHz by the DAC in the YM2612, not by the CPU, and multi-channel 22~23 kHz PCM or DPCM is quite possible even with the Z80 handling the sound chips as well)
Key on any sample (preferably a looping one, as the noise will cut out early if you use non-looping samples), then make sure noise is enabled for the channel to output noise instead of the sample. Don't forget to set its frequency!