Dave,

I am a renter, and all my club's planes have
built-in intercoms without an audio input.

They most likely have it, but the connector wasn't installed. Try
complaining.


I've heard that some headsets have music inputs,
but I have two nice headsets (DC H10.13XL and
DRE6000enr) and don't want to buy another.

Any idea how to do this? Is there a box I can
plug in between my headset and the jacks?

The PS Engineering Muse does what you want.


--
Thomas Borchert (EDDH)