N81

Nokia N81 (Symbian s60 3rd Ed.)

Download the mp3 file to your computer.
On your telephone enable Bluetooth,
Right-click the mp3 file you downloaded and choose “Send to…” and then “Bluetooth device”
Browse for the devices around to find your phone (your phone must be “visible” or “public” to other devices - Bluetooth settings)
Accept the message on your phone, open it, choose “Options,” and “Save.”
You can assign the sound to incoming voice or video calls, message, alarm and email alerts in Profile customization applet.

[edit] ALTERNATIVELY

Download the mp3 file to your computer.
Connect the phone to the PC with the data cable.
If you have Nokia PC Suite installed, a message will appear near your desktop clock that the phone is connected.
Right-click on the Nokia PC Suite icon in the sys-tray, and choose ”File manager.”
Copy (e.g dran-and-drop) the mp3 file into the “Sounds” folder on the memory card or in the phone memory (Data->Sounds)
You can assign the sound to incoming voice or video calls, message, alarm and email alerts in Profile customization applet.