Found this volume mod over at xda-developers posted by bchalk. By default the Samsung Vibrant’s software is set at 88. That leaves a little more room for volume if you so desire. Follow the instructions below if 88 just isn’t quite loud enough for you.
WARNING: 100 IS VERY LOUD!
 SRC Speech RX Volume
5_lvl : 88
Menu -> Key Input
Change to any number greater than 88 (Max:100)
Press back to save.
It will reset after a reboot so be sure to run through it again if you wish to increase the volume level again. There isn’t enough testing done or reports at this time to tell if moving the volume past 88 will damage or blow out your speaker. So as usual if this mod blows out your speaker you are using at your own risk.
You can also access AndroidSPIN forums HERE to talk about this MOD.