Since my early experiments with speech recognition I had been looking out for a decent head set. I was really pleased when I discovered some Logitech headphones at a knockdown price in our local Staples store. I took them home and plugged them in and started talking. And they didn't work very well.
The sound was indistinct and the recognition was very poor. I spent a while fiddling with the settings and re-training but it was nowhere near as good as it used to be. At this point I decided that I might have bought a duff device so I did a little investigating.
Rather surprisingly, results seemed to be equally bad with the headset microphone switched off or even unplugged. After some investigation I discovered the speech recognition software was still using the microphone inside the computer. It seems that it doesn't always automatically select the headset microphone.
However, now I've managed to make the switch must handle the speech recognition works an awful lot better.
A tip, you can disable and enable sound devices by going to Control Panel -> Hardware and Sound and clicking Manage audio devices. This opens up the Sound dialogue. When you disable a device it rather annoyingly vanishes from the list of devices. You can get a device back again, so you can re-enable it, by right clicking in the device window and selecting show disabled devices from the context menu that appears.