Ok...so I've tried other apps that claim to set the in call volume (Volume Control +, Audio Manager, etc...) but the Thunderbolt would still go back to full blast in call volume every time...
I just tried the app "Volume Locker" , and so far after multiple tests, it is locking the in call volume to a low level I set it at when setting it up!!! I even powered off, did a battery pull, and rebooted then tested it again, and it still kept it locked at the 2/5 volume level I set it up at!!! (Can you tell I'm excited? LOL)
Now don't hate me if it doesn't work for you, and I will definitly post back here if my in call volume goes back up again...but so far so good with this app!
NOTE: This is how I set the in call volume (since Thunderbolt doesn't have a setting for that unless you are physically on a call...and the app doesn't let you actually tell it what level you want...it only locks the levels you tell it to that are currently set up on your phone from changing...
Have someone call you, so you can get the option to change the in call volume while on the call (two clicks on up volume, then turn it down to however low you want it...then WHILE YOU ARE STILL ON THE PHONE CALL, hit your Home button...open the Volume Locker app, and click to check Lock In Call volume...and you are done!
(If you don't stay on the active phone call until after you have manually changed the Volume while on the call AND opened the app and clicked to lock the in call volume while still on the phone call, I found it will just go back to full volume...)
I sure hopes this works for everyone else with the same problem as me...I'll definitely post back if it for some reason decides to quit working or anything...