The logical way to do sound would be to pre-process. e.g. for SID emulation, just calculate the resultant sample values for each upcoming scanline with the '816 and have them ready for the 6502 to read via Timer IRQs in the next frame. At 15 MHz, such processing should allow relative HiFi vs existing methods at probably 20% or less CPU expenditure of the '816.
That leaves the remaining cycles for the '816 to do rendering, calculating, other stuff.
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