Here's a video from Mari Smith with excellent tips about online audio enhancement:
She includes here some inexpensive microphones and connectors that will even work with smartphones to bring your audio up to snuff. Other resources for recommended microphones for Webcasting can be found here.
Audio enhances learning outcomes, especially when it involves narration that explains a complex visual, formula, graphic, or video with which students are unfamiliar. Research indicates that adding audio (as opposed to text) to this kind of material can improve learning outcomes by as much as 80 percent. Simply by adding phonetic memory to visual memory, the brain is able to process the information more efficiently. If students are trying to understand a drawing or math formula you've presented to them, adding more text - which increases the strain on their visual processing - won't help as much as explaining to them with audio (your voice), which allows for audio processing to work with visual processing to produce understanding.
Since audio is so important, you as a speaker (presenter, lecturer, etc.) should do your best to optimize the quality of the audio you're providing to your audience. Prepare your environment for any unexpected sound (noise) that will interfere with your narration. Turn off your cell phone (muting doesn't always eliminate all the alerts your phone wants to send you). I use a sign on my door (the current one reads "GET A ROOM...") to let others know you're not to be disturbed. Ask someone to take your dog (or baby) for a while - somewhere else. As e-learning guru Ruth Clark advises: "
Avoid ear candy. Background music and environmental sounds create unnecessary cognitive load and distract from, rather than increase, learning. Indeed, music, over longer periods of time can be incredibly annoying. Note that this also applies to sounds, such as beeps or applause, that reinforce right and wrong answers. This may be appropriate in a games, but not for most online learning. Ear candy is as bad as eye candy."