Remember that the output of an 8051 is basically an open-drain with a high (50 kOhm) internal pull-up resistor. Connect the speaker between the pin and Vcc and you'll get a much higher volume. Do not omit the series resistor! If you do, you will be scraping bits of speaker off the ceiling and you might damage your controller. The resistor should be at least 1 kOhm or so.

Rob.