This is from the Apple pdf (page 24) about the MBP 17 that is downloadable from here
http://images.apple.com/macbookpro/pdf/2006042417MacBookProTO.pdf
"With twice the number of speakers, larger
speaker enclosures, and increased total power,
MacBook Pro delivers double the sound and
plays more low frequencies than the 17-inch
PowerBook G4."
The other specs just refer to inputs and outputs.