This is exactly the issue I have! I got a ringtone sent through MMS, and now it plays every time I open a text (even though I have since deleted the ringtone and original message)!
GOT IT TO STOP!
Try removing the battery & then putting it back in. Read that on another forum and it worked for me.
Well that seems to have worked for now. I would like further input as to why this happens. It almost seems that this could be used negatively and could be quite destructive.
This is one of many known issues with the OS 5.0.0.591 update from Verizon:
http://forums.crackberry.com/f141/known-issues-os-5-0-0-591-verizon-tour-9630-a-446876/
-Severely reduced battery life
-Browser clicking and zooming in Blackberry native browser takes 2-5 clicks before getting a result.
-audio clips received through MMS get "stuck" to the end of ringtone alerts, even if they've been deleted. Only known fix is to do a battery pull.
-Reboot time greatly increased.
- Memory leaks