Fixes a race condition (noted in a comment for wodOpen) when multiple
threads try to open the same wave-out device simultaneously.
Addressed by creating the device mutexes when the driver is
initialized, instead of as each device is opened.  Then use the mutex
to protect the open operation against races.  At the same time, made
the mutexes recursive to avoid self-deadlocks the driver was
encountering when reentered from the callback.