You have been able to connect the computer that will play your PowerPoint presentation to the projector. You have been able to use a VGA connection. You are running a composite audio cable from your "headphone" jack of the computer soundboard to a pair of speakers.
Ready to Go!
But, When you look at the screen, there is no "feed" from the computer. All the connections are solid, but there is no output.Your computer or projector has got issues. The computer has not been told by you to forward a signal to the external vga port to which you have so carefully connected a vga cable. (Signal Feed)
The computer has not been told which "desktop" to display the PowerPoint on. (When you look at what the projector is projecting you can only see the nice background of your computer's main screen (desktop). (Clone or Extended Desktop)