Garmin 340 intercom about 4 years old. I don't think the intercom is
the issue because I have no problems hearing his other headset...

Thomas Borchert wrote:
Jon,


My airplane partner has the Lightspeed Mach 1 headset that fits in your
ears similar to the Clarity Aloft and I can't hear what he is saying....



What intercom? That kind of symptom often indicates a sub-optimal intercom
system.