Ringtone does not play right

[OwenW]

I have used Audacity and the HTC MP3 trimmer to create custom ringtones, and they work, sort of.
Whenever i get a call, the ringtone starts off very faint, and the phone makes a dual-tone sound, then the volume goes up and it plays at normal volume for a few seconds. Then it gets faint again, and repeats the alert sound.
How can I prevent that system alert sound from interrupting my ringtone?
TIA...