I've solved my problem, and feel like an absolute fool! I suddenly remembered that different ringtones could be assigned to different contacts, so I checked the contact who had sent me into a tailspin in the first place, and sure enough, I had assigned 'Ripples'!: If other people are having this problem, as was indicated in the responses, you might want to check this out:
1. Open your Contacts list.
2. Select the name of the contact to customize.
3. Touch the Edit button....
4. Choose the desired ringtone from the list....
5. Press the Done button in the upper right-hand corner of the screen.
6. Tap the Done button again.
I daresay that just about everybody and her mother knows this already, but for those (like me!) who only realized it after a sudden brainstorm, it might prove helpful.