Why does 555 timer output change based on load?
The 555 is only rated to about 200mA input/output, I'm guessing the output current through your 8 Ohm speaker is pulling the voltage down low enough to screw with it's operation. Try a smaller load and see what you get (20mA or so, so try with a 300 Ohm resistor).

If you need more juice use the 555 to drive a transistor.