You can use a mono(male) to stereo(female) adaptor to allow
stereo earphone to work with a mono source.

The Radio Shack 274-368 is one such adaptor.

"Use stereo headset with mono device such as scanner!
* Accepts 1/8" stereo phone plug
* Fits 1/8" mono phone jack
* Lets you connect stereo headphones to scanner,
CB or shortwave radio, or recorder"

Terry