Disable expansion sound channels when PAL emulation is active.

Fix volume levels of expansion sound chips, and see if their DACs exhibit
any non-linearity.  Also implement a global volume-lowering effect for
all expansion sound chips(they have a resistor across the audio in and audio 
out)...

Redesign the FIR filters using software capable of handling such high-order
filters more accurately.

Fix the emulation of the NES' analog sound filters.
