It is important you use a MacSpeech-certified microphone with MacSpeech Dictate. This is because the Mac OS prefers a stronger gain than Windows, so most microphones designed for and tested with Windows may not be as accurate when dictating (this is also why Skype users for Windows sometimes report Mac Skype users are harder to hear). If you purchased your microphone from MacSpeech (either with Dictate, with iListen, or separately) you are all set. If not, you can check the following article to see if your microphone has been certified by MacSpeech:
KB Article 277: "What microphones/headsets are certified for use with Dictate?"
Most Macintosh computers have LINE inputs, not MIC inputs. Nonetheless, a common mistake people make is to connect their microphone directly into the LINE IN port of their Macintosh. This will not produce enough signal for speech recognition, since your microphone outputs a MIC signal, not a LINE signal.
This is why we ship a USB connector with the microphones bundled with MacSpeech Dictate. To ensure your microphone is properly connected, do the following: Locate the microphone plug on your microphone. If color coded, this plug will be pink. Or it may also have a small microphone icon on it.
Locate the microphone input receptacle on your USB adapter. Depending on which USB adapter you are using, this receptacle may either also be pink or it may have a picture of a headset on it.
Insert the plug from your microphone into the appropriate receptacle on your USB adapter.
Some USB adapters also have a receptacle for sound out. This is so you can hear sound coming from your computer through your headset. If you want to hear sound from your computer through the headset, insert the other plug into the other receptacle on your USB adapter. If your USB adapter does not have a second receptacle, then insert this plug into the headphone port on your Macintosh. (NOTE: You may have to set the Sound Output source in your Sound System Preferences before you will hear sound through the desired device.)
Plug the USB adapter into an open USB port on your computer. It is important you plug the USB adapter directly into your computer, since many hubs do not support USB audio.

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