If you haven't downloaded extensions I'd suggest you to do so from here:
http://www.standingwaterstudios.com/
Close Reaper and install them.
Load reaper, load your project and go to Actions->Show action list. In that list you'll find now 'Xenakios/SWS: Reset volume and pan of selected track(s)' and 'Envelope: Clear envelope'. Use these 2 actions to reset your tracks.
NOTE: Don't forget to select the envelope lane before trying to run 'Envelope: Clear envelope'