SetVolumeAlpha.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using PicaVoxel;

public class SetVolumeAlpha : MonoBehaviour {

    public int alpha = 0;

    //Sets the alpha on all voxels in the volume this script is placed on.
	public void SetAlpha () {
        var frame = GetComponent<Volume>().Frames[0];
        for (int i = 0; i < frame.Voxels.Length; i++)
        {
            var c = frame.Voxels[i].Color;  // get the current voxel colour
            c.a = (byte)alpha ;  // set the alpha
            frame.Voxels[i].Color = c;  // set the colour back
        }
    }
}