Root the phone, then delete the audio file. Not sure where it is on the Charge. Try looking in /system for a resources or customizations folder. On stock Android (or AOSP ROM's) it is in /system/media folder. You are looking for an MP3 file. On the DInc, the file names are VZW_droid.mp3. Could be similar.
Settings > Sounds Settings > Volume > turn System Volume all the way down.