Nevím, ale zkusím předběhnout hosta. :)
www.google.com/search?hl=cs&q=sound+notification+Ou tlook+2003&btnG=Vyhledat+Googlem&lr=

/edit: První odkaz říká:

1. Click on Tools, then Options.
2. Preferences tab and then click on E-mail Options…
3. In E-mail Options window, click on Advanced E-mail Options.
4. In the box entitled When new items arrive in my Inbox select the choices that you wish. You may select other options at this time as well.
5. Click OK. Click OK. Click OK for the third time. This gives you your notification of new mail options.

If you wish to change the sound, go to Control Panel (not in Outlook) then click on Sounds and Audio Devices. Select the Sounds tab and scroll down to New Mail Notification and select that. In the Sounds drop down menu below the list of Program Events, select the sound you want from the list or browse and select your own sound file. Once you have it selected, click OK and close the Control Panel.