Improve RT-safety by eliminating some global locks

- I've noticed several places in the code where we're locking the global mixer mutex entirely unnecessarily: there seems to be no adverse effects from removing these mutexes. So I've removed or commented them for now, and tested after each removal that there are no crashes or instability caused by it.
- Also, I modified addPlayHandle() so that it doesn't need to lock the global mutex: instead, it uses its own mutex and adds playhandles first to a temp array, which then gets merged to the main ph-array in a suitable timespot.