The sound of the letter S can be more than a little troublesome for some voiceover talent. Some of us choose to fix this problem with audio processing, though it’s easy to over-do this. On Voice Over Xtra you’ll find an excellent article by Susan Berkley about how to tame this problem without resorting to audio processing.
And if you’re not familiar with Voice Over Xtra, check it out. There’s a ton of valuable resources and membership is free.