If the mic doesn't work, yet it was described as working in the listing, you don't know what other damage it might have had. Has it been submerged in water? You wouldn't know
My advice: send it back. If they then want to claim the microphone repair from the shipping company - that's up to them.
Presuming you don't take that advice, get a bigger discount off the price from the seller. If you end up needing parts - they aren't cheap compared to what you've already paid:
http://www.davidclark.com/store/prod...=8&selection=8
In saying that, most of the DC headsets have a gain control screw on the mic itself - it might be worth playing with it (normally a tiny adjustment produces a big change).