http://jap.hu/electronic/infragate2.html
http://www.reconnsworld.com/ir_ultrasonic_40khzirtest.html
http://circuitos.tripod.cl/schem/r17.gif
http://www.ee.washington.edu/conselec/Sp96/projects/mitchrob/proposal/proposal.htm
produces powerful 1uS infrared light pulses from a low cost LED that are frequency modulated by the audio information.
http://www.imagineeringezine.com/PDF-FILES/voicex2.pdf
a very inexpensive C-MOS IC that is connected to a small photodiode provides high sensitivity under high ambient light conditions
http://www.imagineeringezine.com/PDF-FILES/4069rvr.pdf
you can achieve a range of several hundred feet with a standard TV or VCR remote control module.
http://www.imagineeringezine.com/PDF-FILES/40krvr1.pdf
http://www.discovercircuits.com/PDF-FILES/40kvcr.pdf