Close audio stream when nothing is playing about audioio.jl HOT 1 OPEN

1. You are right, if different sample rates (SR) could be played by the same
   stream there would be no need to open/close a new stream.
   On the other hand, SR conversion adds computational overhead and
   introduces errors for non-rational SR ratios. And as most modern audio backends
   support multiple streams/clients I believe it would be more efficient
   to open a new stream for short audio files.
   For the node (multitrack-like) behaviour of AudioIO, different streams are
   useless because of the synchronisation and latency issues you already mentioned.
2. I am not familiar with the low-level design aspects of julia but I could
   imagine that the less IO-stream overhead the better. I noticed a rising
   CPU workload of julia and portaudio/pulseaudio when a stream is open even
   when no sound is playing.
   For a single audio file, that should only be played once, I think it would be
   more efficient if the stream is closed after the sound ends.
   For a node with multiple events (multitrack) this is definitely not the case.

Therefore I believe it would make sense to differentiate both cases
(single-track/multi-track) and use different approaches for the play function.

Please keep in mind that I used julia and AudioIO on Linux only, so my
observations could be specific to my setup and not an issue on MacOS/Win.

from audioio.jl.