code: protected Source acquireSource (Sound acquirer)
      {
          // start at the beginning of the list looking for an available source
          for (int ii = 0, ll = _sources.size(); ii < ll; ii++) {
              PooledSource pooled = _sources.get(ii);
              if (pooled.holder == null || pooled.holder.reclaim()) {
                  // note this source's new holder
                  pooled.holder = acquirer;
                  // move this source to the end of the list
                  _sources.remove(ii);
                  _sources.add(pooled);
                  return pooled.source;
              }
          }
          return null;
      }
docs: Called by a {@link Sound} when it wants to obtain a source on which to play its clip.