When Outlook receives a new message, a default tone is used. This ringtone is managed in Windows and not in Outlook. It is also used by the Mail app, the default e-mail client (not Outlook) bundled with Windows 1
Turn Off the Beep
If you do not want to hear an audible alarm each time, you will receive a message (which can be particularly annoying if you use headphones). Noise at all. Go to File> Options> Email and scroll down to the "Arriving Message" section. The setting you are looking for is "play sound".
Turn off this option and click OK. Outlook will no longer beep when a message arrives.
RELATED: How to Turn Off New Message Alerts in Microsoft Outlook 2016 or 365
Change the ringtone to something else
If you still want an audible alert but want a different sound than the default one, Open the Control Panel (click on Start and type "Control Panel") and then click on the "Sound" option.
In the following "Sound" window, go to the "Sounds" tab, scroll in the Scroll down the Program Events box a little and select "New Mail Notification." Option
By default, Windows uses the built-in sound file "Windows Notify Email.wav." Open the drop-down menu to select a different installed sound or select "Browse" to select your own sound file.
The sound must be a .wav file, so if you have a different file type you need to use it ie converting it (we're big fans of Audacity or VLC for audio work, but we use your tool of choice). When done, click "OK" to save the changes and close the Sounds window. Restart Outlook and your new sound will play when a message arrives.