Now you open an audio device and attach streams, as planned, but each
open generates a new logical device. Each logical device has its own
streams that are managed as a group, but all streams on all logical
devices are mixed into a single buffer for a single OS-level open of
the physical device.
This allows multiple opens of a device that won't interfere with each
other and also clean up just what the opener assigned to their logical
device, so all their streams will go away on close but other opens will
continue to mix as they were.
More or less, this makes things work as expected at the app level, but
also gives them the power to group audio streams, and (once added) pause
them all at once, etc.
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