Make sure you have the info line visible at the top of your timeline. There you will see a box with "Volume" in it, and underneath that the actual volume of the selected clip. Just CTRL-click the value and it will revert to 0.
In Nuendo, the info line is a godsend. You can change plenty of clip parameters just with the scroll wheel (like clip length, in fade, out fade, transpose etc).
Also note that this volume is completely separate from the automation volume (track volume as opposed to clip volume), as well as from the event envelope. That makes 3 ways of adjusting levels for a sound in the timeline. Most of the time during sound editing I will only use the clip volume and envelope, leaving the faders for the mix.
Paris, France Heavy Nuendo users
"I don't care what they're talking about, I just want a nice fat recording" Harry Caul
My blog: http://www.filmmixer.eu