I found this on another forum. It's for the Vibrant, but works for the Captivate just the same. I haven't personally tried it as I am afraid of blowing out the speaker, but if I could get the speakerphone to be a little louder it would be nice - may try it later...
How To: Increase Your Samsung Vibrant’s Volume Beyond Full ? AndroidSPIN | Your No.1 source for Everything Android.
How To: Increase Your Samsung Vibrant’s Volume Beyond Full ? AndroidSPIN | Your No.1 source for Everything Android.