The sound of silence

Quick update; added sound hooks and osd stub functions for them - but nothing else!

Have been doing a little more RE of the SOUND CPU and it has helped a little, although I won't be needing (or wanting) to complete it. It's a bit more complicated than I first imagined, with lookup tables for which sounds to silence when certain other sounds are playing, for example. I don't think we'll go that far into replicating the behaviour - it has probably more to do with hardware limitations than anything else anyway.

Still yet to look for that last gameplay bug...

...but I was playing it tonight and when the Bacura appeared I was waiting for the bug to manifest itself. It wasn't happening when it should have... and then I realised I had actually fired up the arcade original to check out some of the sounds! I guess it's a good thing when you can't tell the difference between your port and the arcade game (ignoring the obvious aural hints!)