Increase the volume on your Samsung Vibrant
To raise the volume on the Vibrant, you will need to punch in some code numbers on the dialer after you have raised the internal volume controls to your desired level. Complete directions can be found at the source link. It is not known whether or not this process will work on the Vibrant's sister phone, the AT&T Captivate. Both are versions of the Samsung Galaxy S but so far, there have been no complaints about the speaker on the AT&T variant.
Samsung Vibrant Specifications | Review
Samsung Captivate Specifications | Review
source: Xda-developersforum via AndroidGuys