I asked a similar question to this before... so allow me to answer.
The 15" Powerbook has a headphone port. The 17" Powerbook has digital audio in/out. So the 17" Powerbook can handle a 5.1 system, but the 15" needs something like
this to work.
As far as a good speaker system, I am planning on buying the
Klipsch Promedia Ultra 2.0 because they are great desktop speakers that look cool with a powerbook. Plus I don't need another sub.
Klipsch makes awesome looking and sounding 2.1 and 5.1 systems, as well.