Right click on your desktop and choose Personalize.
Left click on Sounds at the bottom of that window. (Keyboard and musical notes icon).
Click the Sounds tab in the box that pops up and in the center of that box you will see a scrollable window.
Scroll down to New Mail Notification then highlight it by clicking on it once. That will allow you to change the sound in the bar below it. Click on the Browse button and direct it to where you saved the TunnelTap sound. Click on OPEN and it will show in Sounds bar. Test it to make sure it is good, then click Apply, and you should be good.
Play with it and you will find that you can assign all kinds of different sounds.
Janet
------------ *miau* "i´m team self-serving psychopath - a.k.a. DAMON" ------------