This may sound daft but singers are often advised not to eat dairy products before a concert because the dairy lubricates the throat with phlegm and takes away a certain amount of crispness from your voice. A stuffy nose from milk allergies etc will make you wound nasal and terrible in voice recordings.
Additionally, as Brian correctly suggests below, enunciation is critical. If you watch video broadcasts of television or radio hosts you will see that their enunciation seems exaggerated - their mouths seem to move more than in everyday people's conversation. It may feel awkward at first, but we are all a bit lazy in how we move our mouths to make words crisp and clear. After a while, enunciating clearly will become habit but you can practise using the record function on your mobile phone and record snippets trying different techniques to make your speech more clear.
Learning to speak more crisply in particularly helpful in an audio clip because you do not have the benefit of seeing your mouth move and as such, the lip-reading element which is a natural part of comprehending human speech, is removed and your speech needs to be particularly clear.
Hope this helps a bit...