You can, of course, speed up and slow down messages being played back in your inbox from over the phone (or via the media master at your desktop). This, however, has no effect on the system prompts which is what you're asking about.
You can speed up or slow down the prompts system wide (this affects everyone, not just a single user) with the Set Prompt Speed tool - you'll find this in the Tools Depot on your Unity desk top under the Audio Management section. You can also download the latest from its home page here:
http://www.ciscounitytools.com/App_SetPromptSpeed.htm
Just be aware that it affects all callers (internal and external) - and maybe not everyone has had as much coffee as you so be gradual with any changes. Also, be sure to backup your prompt set before touching it as instructed in the help (it has a mechanism for doing this for you automatically).
