basically, you want your buffer rate to be as to-the-point of no under-runs and under-runs.
right now I don't remember if you want the buffer rate higher or lower, but make it go to one side, and if the underuns go up dramatically, then keep sliding the rate until you have none. (assuming that your sound driver has a slider for this.
not sure if that explains what it is though, so here it is.
buffer rate is the rate at which the soundcard buffers? idk here's what wikipedia says:
The buffer in an audio controller is a ring buffer. If an underrun occurs and the audio controller is not stopped, it will keep repeating the sound contained in the buffer, which may hold a quarter of a second. Such effect is commonly referred to as "machinegun". This happens if the operating system hangs during audio playback. An error handling routine (e.g. blue screen of death) may eventually stop the audio controller
so that's why you need to set it just enough so that no underruns occur, because if the rate is too high (I think) and doesn't need to be, it just wastes CPU.
hope this helps XD havent had to deal with buffer or CPU problems in over a year yet, so my minds a little bit confused XD