code: protected void baseGainChanged ()
      {
          // notify any sound currently holding a source
          for (int ii = 0, nn = _sources.size(); ii < nn; ii++) {
              Sound holder = _sources.get(ii).holder;
              if (holder != null) {
                  holder.updateSourceGain();
              }
          }
      }
docs: Called by the manager when the base gain has changed.