How can I hook up a simple MOSFET to amplify the PWM signal from Arduino?
Why dont you do a complimentry drive for your proposed book up .If you do this then you can capacitively couple your speaker.Remember that the DCR of a 8 ohm speaker is approx 6 ohms . This rough ratio doesnt change much with speaker impedence.You could connect a couple of logic level fets in the CMOS configuration or you could use a grunty mosfet driver chip that say has an on resistance of 1 ohm.