If Dragon Professional Individual misinterprets lots of words, your second thought is probably, "Something's wrong with my microphone or sound card." (Your first thought is probably uncharitable toward your new assistant. Shame on you.)
Dragon Professional Individual may be messing up for lots of reasons, however. As with a puppy that messes up, one reason may be inadequate training. Before you go looking for an audio input problem, make sure you have done some additional training. One symptom of a need for additional training or vocabulary building is that Dragon Professional Individual makes the same error repeatedly: "cheese" for "trees," for instance.
If, however, Dragon Professional Individual gives you different text each time you say the same thing ("wheeze," "sneeze," and "breeze" for "trees"), you may have an audio input problem. One dead-simple way to test for serious audio input problems is to listen to your PC play back your voice. You can use a Nuance-approved sound recorder, if you have one, or you can use the Windows Sound Recorder:
For Windows 7, access the Sound Recorder by choosing Start→Control Panel→Sound→Recorder tab. Click the red-dot button in Sound Recorder and speak into the microphone to record. Click the red-dot button to stop and save the recording to a location of your choice.
For Windows 8, access the Sound Recorder by going to Search on the Charms bar and typing Sound Recorder. The recorder appears on the left side. Click it to show the recorder. To record, click the red-dot button in Sound Recorder and speak into the microphone to record. Click the red button to stop and save the recording to a location of your choice.
For Windows 10, access the Sound Recorder by going to All Apps and then clicking Voice Recorder. A screen opens with a microphone. Click the microphone to start and then click again to stop, and save the recording to a location of your choice.
The most common cause of recognition errors is muttering through your coffee cup, or otherwise not speaking clearly. Does your diction qualify you for the British Broadcasting Company?