I'm assuming that you have an older dual USB iBook where there is an adaptor for VGA which plugs into the port between the USB and heaphones and another adaptor for both audio and video which plugs into the "headphone' port and splits to 3 phono ends.
I think you're having trouble because you're plugging both adaptors in, the VGA for video to the projector and the composite for audio to the speakers. The iBook might be confused because of both these adaptors being plugged in.
Try connecting the VGA to the projector and using some headphones to check for sound. If this works then you'll need a cable like the compostie adaptor, but with only 2 phono ends for the audio. You can get these at any store like Radio Shack.