I had the same problem with incoming calls not ringing on the headset or on the PC speakers. After several weeks of back and forth with Skype tech support, I finally found a technician who had a viable answer. Here are the steps he told me to follow -- and they worked in restoring the ringtone.
1. Quit Skype. Right click on Skype icon on system tray and choose "Quit".
2. Press "Windows key" + R on your keyboard in order to open Run window and type in shortcut command:
3. Once Skype folder is opened please delete "shared.xml" file.
4. Then open your account folder and delete also "config.xml" file.
After deleting shared.xml and config.xml files please uninstall Skype as
1. From Start menu, click on "Control Panel" and then choose "Add/Remove Programs".
If you are using Vista then: Start > Control Panel > Programs > "Programs and Features".
2. Select Skype from the list and click on "Remove".
3. Go to C:\Program Files and delete the "Skype" folder from there.
4. Go to C:\Documents and Settings\All users\ApplicationData and delete the "Skype" folder from there.
For Vista: C:\ProgramData\
5. Open Start menu and select "Run". ("Windows key" + R) 6. Enter "regedit" in the text box and click "OK".
7. Open the "Edit" drop-down menu and click "Find".
8. Search for the keyword "Skype". Delete all search results.
After complete uninstallation please download and install the latest version of Skype from: Skype 4.1 for Windows - Download the latest version of Skype and get free computer calls
* Choose "Save" not "Run/Open"