The 68K version plays only mono, 8 bit sound. If you listen through stereo speakers, you'll hear the same thing on the left and right channels.
In contrast, the PowerPC version always uses 16 bit samples, and can be configured to play in either stereo or mono via the "SurroundSurf" checkbox in the preferences dialog. When unchecked, the same sound is played over the left and right channels. But, when the "SurroundSurf" box is checked, SurfSim uses stereo to create the auditory illusion of waves washing through the space in front of you. Note that, to hear SurroundSurf properly, you must listen through headphones. The effect is ruined, and the sound quality takes a big hit, if the two channels mix before reaching your ears. This means that if you listen to SurfSim through external stereo speakers (or through the Mac's internal speaker, which automatically mixes stereo down to mono before playing), you should make sure that "SurroundSurf" is off.
Also, the sound quality of the native version is a bit better overall relative to its 68K cousin, for a number of reasons.
