Okay, here is a more appropriate circuit.
The MOSFET I used is IRF3707Z, any higher current logic level MOSFTETwill work, such as the sparkfun sells.
The speaker is from mpa.com
http://www.mpja.com/prodinfo.asp?number=14618+SP
90dB. I use it in a fencing club to make a warble when a touch is scored, can be heard very well.
The current thru your BJT is (Vsource - Vce)/(sum of resistance in the rest of the path)
So (5V-0.5v)/(8ohm+pot)
