I've listened to it on very high quality hifi speakers and I wouldn't say there was distortion as such. The sibilance is a bit excessive, though it doesn't show up particularly on a spectrum analyzer. If you can tone-control the sound a bit it might help. I've had a go at it by pitting a 9dB notch in at 9kHz - there's still a bit of an audible peak on the sibilance but reducing it any further will make the voice sound a bit muffled. I'm afraid that the probably cause is that the microphone is designed with this toppy sound because a lot of people think it's good - though I don't. Your best bet is to see if you can use a graphic equalizer program or plug-in and see if you can get the sound to your liking.
You can download my revised version at http://rfwilmut.net/share/voiceover.mp3
<This link is a direct download>
As a check on your speakers etc. you might like to listen to a bit of my podcast at http://rfwilmut.net/podcasts/podcast30.html - this is fairly good speech quality from a cheap Beyer moving-coil microphone in lounge, slightly sibilant but nothing like as much as yours.