And very common!

Intermittent faults will often show up with a check for "full and free
movement" in all the connections. Just like when you check the controls
pre-flight, you can be gentle but exercise full movement range when checking
them.

You can use just about any old aircraft or CB radio to test the headphone
with an adaptor or two. Even a cheapshop $5.00 FM radio will give an audio
output and can check the ears. The mike circuit could be checked with a
cheap walky talky with a couple of connecting leads to adapt the plugs, or
you could pick up a cheap handheld for the office. Several flight schools
down under have a radio in the "waiting lounge" and this helps your students
to pick up typical radio chatter. Also lets you know what your students are
up to! A tape recorder next to it can catch their circuit chatter and can
offer an additional training aid.

Hope this helps,
Peter