okay, so i've noticed a whole bunch of people have looked ur entry over and as i feared, no one has really come across this problem. Well, neither have i but i want to suggest a few solutions cause i feel bad when people don't get their questions answered.
When it comes to issues such as yours, its really hard to suggest a course of action because it's very much of a vague description you gave. Now, i do understant that u did ur best but when to comes to problems like these, they would need to be studies further and examined up, close and personal, which is imposible by anyone here.
So, here are a few trubleshooting options.
1) have you tried setting a music mp3 as a ringtone just to see weather it does the same thing. I don't mean an mp3 "ringtone" but just any old diddy to see if it's a, ringtones specific problem, a speaker volume problem or a software "escalating volume" issue. It may be that ur speaker is damaged or u have a crappy (old) version of the software. It could be a bunch of things.
2) when it comes to BB's too, i've noticed that they behave very differently from one another. So, perhaps maybe whom ever put on that software prior to you, may have put it on incorrectly or the software just had it's self a "bad install". I'm experiencing that right now with my Mac OS on my apple. Though different platform, an OS is an OS. Every once in a while, even though u follow all instructions, it will somehow install its self incorrectly. So as number two, i suggest backing everything up, and reinstalling the OS software. And maybe u may want to change the version as well. Maybe downgrade or upgrade, just too see if it keeps doing it. And along with a fresh copy of the OS, ur ringtones will get a fresh install as well because they may have been damaged too. Along with the controllers that control the loud speaker volume and the "escalating volume" and ect...see what i'm getting @? it could a million things.
3) if it keeps doing if after u've re-installed the os OR u don't know how to upgrade or downgrade system software, then just take it in to whom ever sells BB's in your city or town ie, Rogers, or T-mobile or Orange...and simply hit up a sales staff joe or a tech for some quick question/suggestions. Ur probably not the first one to experience it, even though it seems like it on this forum.
Ud be surprised how much they know.
thats it really.
hope this helps