You can copy systems sounds to the Outlook Resources folders and rename .aiff to .wav and set the name, e.g. newmail.wav, reminder.wav, mailsent.wave (Purr.aff to newmai.wav / Submarine.aiff to Reminder.wav)
/System/Library/Sounds
/Applications/Microsoft Outlook.app/Contents/Frameworks/OutlookCore.framework/Resources
Because its just sounds cooler.