The microphone is the largest problem that speech recognition encounters. Microphones inherently have the following problems:
Not every user has a sound card. Over time more and more PCs will bundle with a sound card.
Not every user has a microphone. Over time more and more PCs will bundle a microphone.
Sound cards (being in the back) don't make it very easy for users to plug in the microphone.
Most microphones that come with computers are cheap. Because of that, the microphones don't do as well as more expensive microphones that retail for $50 to $100. Furthermore, many of the cheap microphones that are designed to be worn are uncomfortable. A user will not use a microphone if it is uncomfortable.
Users don't know how to use a microphone. If the microphone is a worn on their head they often wear it incorrectly, or if it sits on their desktop they will lean towards it to speak even though the microphone is designed for the user to speak from their normal sitting position; After all, everyone has seen rock stars on TV, and they are practically eating the microphone.
Most applications can do little about the microphone. Luckily, when the user installs the speech recognition engine, the engine should come with software that makes sure the user's microphone is correctly plugged in and working.
Most users of dictation will wear close-talk microphones for maximum accuracy. This alleviates a number of problems that Command and Control recognition encounters because the user is wearing a headset microphone.