Try this....
1. Start Skype and log in.
2. Click on "Tools" (at the top of the window) and then click "Options" from the drop-down menu.
3. In this new window click "Sounds" and then click the "Import Sounds" button.
4. Now you want to locate and select your sounds using the file browser from this screen.
IMPORTANT NOTE: All sounds must be in WAV format. So if you are using any other file type, you will need to convert it to a WAV file (and WAV files are quite large).
5. Click on "Open" to import the sounds into Skype.
6. Now select the first sound you want to change from the "Select Which Events Play" a Sound list. The current sound is displayed in the "Choose Which Sound to Play" box.
7. Click the "Choose Which Sound to...