Are you using BerryBuzz? If so, go to: Options -> BerryBuzz -> Battery Fully Charged and change "Alert when fully charged" to no.
If you still want a sound when fully charged, but would like to change the notification, go to: Profiles -> Ring Tones/Alerts -> Berry Buzz-Battery Fully Charged Alert.
I note that even when I have the volume set to "Silent," I still get the sound when docking or plugging in. The only way I found to make the sound go away is to turn it off ("No") in the Options - > BerryBuzz area as mentioned in my first paragraph.