The sampling rate for the Macintosh IIsi is set at 11khz by the software
that comes with the Macintosh IIsi. Third parties will be delivering
software that will take advantage of the greater sound capabilities of the
Macintosh IIsi in the near future.
There's no way for you to upgrade the Macintosh IIsi sound chip as it is
soldered to the logic board. Any upgrade has to be done by Apple at the
factory.
In terms of hearing a better output, you hook up your Macintosh to a
stereo system, or simply attach some speakers to the Sound Port on the
back of the Macintosh. Bose is one company that makes speakers that are
popular with the Macintosh.