Among the common causes of camera damage is the operator walking away from the tripod with headphones still on. IR headphones can be useful on set as well as at home or in the car, too. They have excellent sound reproduction and prices are dropping.

For location work, the usual 6 meter range is close enough from camera to headphones, but the mic and transmitter may be further away from the camera.

Look for at at least 10 meter headphones on Ebay

Transmitters often come cheaply and with better range. I got a 12V, 10 meter model for $12.00. Look on Ebay too.

Transmitters and headphones can be bought together as a set. Of course, several crew members can receive the same sound via phones.

This is just one more example of domestic equipment which has improved (for in-car DVDs) so much that in can trickle-up to pro applications. Of course, if you're like me you'll find domestic uses as well. (Think kids, peace at home, long drives...)

There are some domestic and car appliances (eg NextBase) which already include IR transmitters. Almost all seem to share the standard stereo IR transmission frequencies of Right: 2.8 MHz. Left: 2.3 MHz.

You can get IR receivers, too. The old Sony 6V IFT-R10 also comes with good quality composite video as well as line-level audio suitable for monitoring. Maybe somebody has experience with camera-mounted IR receivers for audio signal input.