This plugin for MuseScore 2 randomizes the velocity of notes between minRandom and maxRandom

randomvelocity.qml

//=============================================================================
//  MuseScore
//  Music Composition & Notation
//
//  Copyright (C) 2018 Nicolas Froment
//
//  This program is free software; you can redistribute it and/or modify
//  it under the terms of the GNU General Public License version 2
//  as published by the Free Software Foundation and appearing in
//  the file LICENCE.GPL
//=============================================================================

import QtQuick 2.0
import MuseScore 1.0

MuseScore {
      version:  "1.0"
      description: qsTr("This plugin randomizes the velocity of notes")
      menuPath: "Plugins.Notes.RandomVelocity"

      property int minRandom : -10
      property int maxRandom : 10
      
      // Apply the given function to all notes in selection
      // or, if nothing is selected, in the entire score

      function applyToNotesInSelection(func) {
            var cursor = curScore.newCursor();
            cursor.rewind(1);
            var startStaff;
            var endStaff;
            var endTick;
            var fullScore = false;
            if (!cursor.segment) { // no selection
                  fullScore = true;
                  startStaff = 0; // start with 1st staff
                  endStaff = curScore.nstaves - 1; // and end with last
            } else {
                  startStaff = cursor.staffIdx;
                  cursor.rewind(2);
                  if (cursor.tick === 0) {
                        // this happens when the selection includes
                        // the last measure of the score.
                        // rewind(2) goes behind the last segment (where
                        // there's none) and sets tick=0
                        endTick = curScore.lastSegment.tick + 1;
                  } else {
                        endTick = cursor.tick;
                  }
                  endStaff = cursor.staffIdx;
            }
            console.log(startStaff + " - " + endStaff + " - " + endTick)
            for (var staff = startStaff; staff <= endStaff; staff++) {
                  for (var voice = 0; voice < 4; voice++) {
                        cursor.rewind(1); // sets voice to 0
                        cursor.voice = voice; //voice has to be set after goTo
                        cursor.staffIdx = staff;

                        if (fullScore)
                              cursor.rewind(0) // if no selection, beginning of score

                        while (cursor.segment && (fullScore || cursor.tick < endTick)) {
                              if (cursor.element && cursor.element.type === Element.CHORD) {
                                    var graceChords = cursor.element.graceNotes;
                                    for (var i = 0; i < graceChords.length; i++) {
                                          // iterate through all grace chords
                                          var graceNotes = graceChords[i].notes;
                                          for (var j = 0; j < graceNotes.length; j++)
                                                func(graceNotes[j]);
                                    }
                                    var notes = cursor.element.notes;
                                    for (var k = 0; k < notes.length; k++) {
                                          var note = notes[k];
                                          func(note);
                                    }
                              }
                              cursor.next();
                        }
                  }
            }
      }

     function getRandomIntInclusive(min, max) {
           min = Math.ceil(min);
           max = Math.floor(max);
           return Math.floor(Math.random() * (max - min +1)) + min;
           }      
      
      function randomVelocity(note) {
            note.veloType = Note.OFFSET_VAL
            note.veloOffset = getRandomIntInclusive(minRandom, maxRandom)
         }

      onRun: {
            console.log("hello random velocity");

            if (typeof curScore === 'undefined')
                  Qt.quit();

            applyToNotesInSelection(randomVelocity)

            Qt.quit();
         }
}