When recording video, sound is just as important as the visuals. Good audio is incredibly important for video, like audition tapes, web series, or recordings of live performance. Unfortunately, bad audio is much more noticeable than good audio, but high quality audio can be achieved with just a few tricks.
Recording audio while shooting an audition tape will be the easiest of the three since, chances are, you'll be by yourself inside. If you're in a quiet room and using a video camera, you'll be able to use the built in microphone in the camera. Make sure to adjust the microphone sensitivity for the amount of volume. If the microphone sensitivity is set too low, the recording will be too quiet. If the sensitivity is set too high, loud noises will cause clipping (also known as peaking). Clipping occurs when the microphone picks up more volume than it can handle, which causes the recorded audio to sound distorted. Cameras with built in microphones will have a volume meter that displays when the audio is peaking (often signified by the meter "going into the red"). To monitor your volume and to make sure the audio doesn't peak, always wear headphones.
When shooting a web series or a short film, you'll want to designate a "sound guy," who can hold the microphone and monitor levels. You'll also want to buy a separate shotgun microphone, which will allow you to get the microphone as close to who ever's talking as possible. Shotgun microphones are condenser microphones designed to record live sound. These microphones are very directional, meaning they're very good at isolating sound directly in front of them. When recording with a shotgun microphone, position it as close to the source of sound as possible without entering the camera's view. This is often accomplished by attaching the microphone to a boom pole.
Live performances are the most challenging to record sound for, since you'll only have one shot to get the sound right. Make sure to position yourself in the center of the audience, a few rows back from the front row. Getting your levels right before the performance starts is tricky, so test during sound check or during an orchestra warm up. Again, make sure to wear headphones and watch the levels throughout the performance. You may need to adjust the levels for particularly loud or quiet scenes.