Calendar alert sound on the iPodTouch isn't loud enough is there some "mobile" way to make it louder?
As far as "volume boost" or "overdrive" type features are concerned, I don't think there is one. You could always get a battery powered external speaker.

When I'm out driving I usually plug my iPhone into my car stereo and listen to music / Pandora. It quiets the music (but doesn't pause) to play notification sounds, so it's quite audible.