This is an unsupported hack, so if your pc bursts into flames as a result please don’t call us. But it can be done and it probably won’t cause any problems.
First you’ll have to create a wav file from the audio clip. Yahoo has a guide on converting an mp3 file to wav, you’ll probably want to follow that. You may want to use a wave editor to reduce the length because the ring will continue after you’ve picked up the call. For a normal song you’ll find this very annoying.
You should exit SightSpeed after that. Then open C:\Program Files\SightSpeed\sounds in Windows explorer. Rename ring.wav to ring_backup.wav. Copy the wav file you’ve created to this folder and rename it to ring.wav.