Plantronics USB DSP-500 headset on the Intel iMac

The headset itself works, but I can't adjust the volume. The inline volume control changes the volume of the iMac built-in speakers, but has no effect on the headset, likewise for System preferences. I went to the Plantronic's web site, but the only drivers they have are for Windows. Any ideas?

 20" Intel iMac + Cinema Display + Sony HDR-HC7 HD camera Mac OS X (10.4.10) All tools, no talent!