Plantronics Audio 480: earbuds work, but not mic

This week bought a Plantronics Audio 480 earphone/mic headset for my brand new iMac. The box hinted that it was a USB, and the manufacturer hid the actual plugs in the packaging, but I was okay with whichever configuration. I was mildly surprised to see a bundled mic plug and earphone plug, of the consumer-grade size. No worries: I plugged them in to their respective jacks in the iMac. I got earphone sound, but no mic sound. I got a store exchange, and the new ones behave no better, which makes me think both were manufactured fine. There's a mute switch on the cord, and I tried it in both positions; no sound. I know how to choose between internal and external mics in System Prefs; the internal mic gave me a level, but the Plantronics mic never did (though when I pulled the plug from the jack it made a noise that I couldn't hear but registered in the level meter). The box sports a Mac OS logo, and it says it's Skype approved. I tried yelling into the mic during a recording in GarageBand; I got a flatline. What else can I do to troubleshoot this thing?