On my 17" MBP the speakers sounded extremely crisp and clear and went up to a surprisingly high volume all without any distortion. General consensus though is that the 15" MBP speakers don't sound as good as they're smaller and I'd assume RELATIVE to MBP's the MacBooks are even poorer. Whichever one you get don't expect any bass cos you won't get any. Best thing to do is go listen to them in store though.
As for whether the MBP is worth the extra 1k. The MacBooks and MBP's are very similar and both very capable machines; these are the only real advantages of a MBP:
Higher resolution and larger screen
Matte option for the display
Backlit keyboard
More ports e.g. FirewWire 800 and ExpressCard slot
Dedicated graphics card - a MUST for any gaming or 3D applications
Ability to connect to a 30" monitor at a 2560x1600 resolution
Basically if you don't need the superior graphics card, just get a MacBook and use the money saved to easily install a large 7200rpm hard disk drive (they're easy to do on MacBooks compared to MBP's) and get an external 23" monitor.
Of course if you'd told us what you actually need to use your laptop for then it'd be far easier to decide!
