Can you download anything else from the iTunes Store or download other iPhone/iPod updates?
Try looking for and removing the VoiceOver kit as shown below.
For Mac OS X:
- Quit iTunes.
- In the Finder, choose Go > Go to Folder.
- Enter /Library/Application Support/iTunes and click Go.
- Drag the "iPodVoiceOver.framework" folder to the Trash.
- Open iTunes.
- Sync your iPod to iTunes.
After removing it, are you able to get iTunes to download and install VoiceOver on your iPod shuffle?