Sound mixer and synth fix by @jpfli. I have tested the audio both in Wins Simulator and HW after the fix using the programs: "PlaySfx" and "SfxEditor".
Refactored to use common audio interrupt implementation for HW and SIM. That is to prevent the implementations to go different paths in the future any more.
Fixed a couple of other problems when compiling under Wins Simulator.
Changes: