code: public void setBaseGain (float gain)
      {
          if (_baseGain == gain) {
              return;
          }
          _baseGain = gain;
  
          // alert the groups that inherite the gain
          for (int ii = 0, nn = _groups.size(); ii < nn; ii++) {
              SoundGroup group = _groups.get(ii);
              if (group.getBaseGain() < 0f) {
                  group.baseGainChanged();
              }
          }
      }
docs: Configures the base gain (which must be a value between 0 and 1.0) which is multiplied to
  the individual gain assigned to sound effects (but not music).