Speakable Items won't work with my headset

Question

Here's the deal:
When I am using the internal microphone with Speakable Items, everything works fine. (I have to lean in a bit, obviously.) But no matter what I do, my computer won't understand commands spoken through my bluetooth headset. My OS is up-to-date (10.4.10).
The headset (Plantronics Voyager 510) pairs fine with my computer, an iMac G5, and appears to work perfectly well as an input device: the readout in the Sound preference pane shows that the computer can hear my voice through it. In fact, when I go to the "Speakable Items" tab in the Speech preference pane, choose the headset (which appears as "PLT 510"), and attempt to calibrate it, the blue, green, and red readout in the Microphone Calibration window also registers my voice.
But no matter where I set the slider, no matter where I am in relation to the headset microphone, and no matter at what volume or speed I speak, the computer will not recognize commands. It can hear my voice — which I keep in the green zone — but won't understand it.
It's not just in the calibration window: it also won't work when trying to use Speakable Items in, say, the Finder. I've tried changing the input volume for the headset in the Sound preferences, having my room totally silent (including shutting off the air conditioner), calibrating with Speakable Items set to "on" and "off," and weeping and begging. I even rebooted. Nada.
Again, Speakable Items works fine when I am using the internal microphone. The one other difference I found in their behaviors is that, with the calibration slider set to low and the room silent, the internal microphone is steady at the second green bar; but the headset under the same conditions is steady at the sixth bar. In fact, in the case of the headset, moving the slider has no effect at all: it always rests on the sixth bar.
(In other news, the headset won't pair with my iBook G3 [Dual-USB] using the D-Link DBT-120 [Rev. B3]. But I guess that's a different issue altogether.)
Googling, I see plenty of happy Mac users who own my model of headset, but I can't find this problem addressed anywhere. I've looked through the forums here as well.
Thanks for reading! Can anyone help?

Answer 1

Sorry for the slow reply, Philip!
First of all, Macs do not yet interact in stereo with Bluetooth headsets, though according to rumour this will be available in Leopard (along with everything else).
Have you tried using the Skype Test Call feature? This is one of the easiest ways of listening back to what you record. Come to that, does it record into one of the more standard sound apps (e.g. Garage Band, Audacity, SndSampler)?
My wife has a Plantronics 590 Bluetooth headset, but when it is interacting directly via the Mac's Bluetooth she can only get the microphone to work with Skype and the Finder. And iTunes outputs only the left-hand channel, evenly split between the two channels, and the quality is horrible. If she wants to use it for any other purpose she has to connect it via the Bluetooth 3.5 mm jack adapter plugged into the headphone socket, which cuts out the headset mike as well as the internal speakers.
Though VoiceOver does work through her headset, without using the adapter, the quality is awful and VoiceOver keeps crashing. She can, however, use it for Speakable Items, though she often has to repeat herself. The calibration had to be set very high — 100% mostly.
Have you gone through the full calibration sequence, including getting all the test commands to flash?
Keep us posted!
Archie