Unfortunately, I do not have the microphone that came with MacSpeech any longer; only this Sennheiser microphone.
I'm not running any of the background software/processes you noted.
I have thought about anything and everything that might be going on in the background with relation to this underlying conflict.
My setup is pretty simple, as far as software is concerned: I just have Safari, QuicKeys, Microsoft Excel, and Photoshop running in the background (meaning, the vast majority of the time). That's it.
At present, the microphone seems to be working normally. No noise, no static, no interference. But this could change at any minute… usually does.
This iMac is fairly new (less than a year old), so I'm thinking it's probably not the iMac itself, i.e. the USB port itself.
If you have any other thoughts/ideas, please pass them along. I will attempt to hook up another microphone situation as soon as I can in order to rule out the Sennheiser microphone and/or the USB connector.
(BTW, I plug the Sennheiser microphone into this USB pod/adapter: http://www.andreaelectronics.com/Buy/ProductDesc/USB_Adapter.htm) The representative of the small company that sold me this new USB adapter said he has never seen one of these go bad–never. So he highly doubts it's the USB adapter.
Thanks for any and all input!