not nessesarily meaning you need a new one, could be some compatibility issue with your card, if it works fine on other games I would not even worry about it, as 2d should still sound somewhat decent.

The sound buffer changes the buffer size, when a sound is played, it has to be buffered into memory first in order to play it without having any skips ect.., it is then accellerated from memory through the audio processor. So increaasing makes more room for sound, but setting to high can result in teh same sound problems, so 400 is probably the "optimum" setting.