Thanks for the responses, but hardware aside what calculations would you recommend for similar style data sheets? Using amplifiers, usb speakers, or any other different hardware is not relevant for this example.
On the arduino site, the 'Tone' tutorial example (http://arduino.cc/en/Tutorial/Tone) uses a similar setup- ardiuno + 8ohm speaker. They recommend a 100ohm resistor. I just want to know what steps they would have taken to work out the resistance?