fpga4fun.com

I can help you out with getting this working.

Get a really good FPGA board, with high speed ADC and DAC. Connect a 3.5mm stereo plug to the ADC input - this goes into the laptop's audio out. Then program the FPGA to sample the signal and pass the samples on to the DAC. Then wire the DAC to a 3.5mm stereo plug, which goes into the projector. If you like you could also do some DSP in the middle, to cancel out any odd room acoustics, or to add reverb or so on...

Should you have any trouble with getting this solution working, send me the FPGA board and associated bits and I will send back a working solution, I've got a special machine that will let me include the FPGA board within the cable, so you won't even know that it's in there!