I would first work through the usual 5 R's of troubleshooting to rule out any sort of possibility of a software/glitch or corrupted data.
It wouldn't hurt to also try another set of headphones just to be sure.
Okay- found out how to fix it. Already listed within the community but not directly referenced to the shuffle 4G. Basically go into the speech control panel on your mac and speed it up. Next time you sync your shuffle the voiceover that's generated is faster.