anchor: Source:getFreeBufferCount
  Gets the number of free buffer slots in a queueable Source. If the queueable Source is playing, this value will increase up to the amount the Source was created with. If the queueable Source is stopped, it will process all of its internal buffers first, in which case this function will always return the amount it was created with.
  buffers = Source:getFreeBufferCount()
  buffers number How many more SoundData objects can be queued up.