RealtimeKit and the audio problem

It's a scheduler issue. An operating system built with such requirements in mind can do the job very well indeed -- but the CPU scheduler has to be designed with the scheduling constraints of the audio and video IO in mind. This is definitely not the case with Linux or most general-purpose operating systems: worse is better is bad for some workloads ;-)

For an example of an operating system that does it 'right', see http://lwn.net/Articles/270081/.