With (currently) having three mixed sound sources, is there a mixer setting, or are they just added on top of each other (or averaged) ?
Also, it may come in handy, that if you do more than one sound source, or especially when you only do ONE sound source (like the Vera), to add a sound-in plug and mixer, so that the expansion slots can be used for sound boards (like a dual-SID expansion board using the ARM-SID) hat can either internally with a cable to a header, or externally be mixed into the sound output. It saves an external mixer panel and allows for a SID Player ROM on the cartridge.
And going a little off-topic (sound) here...
A graphics board design with (with HDMI for example) using dual-access RAM for direct pixel access and more memory/resolution/etc (like AMIGA retarget-able graphics) would obviously be connected separately anyway, but another nice option if you include BASIC ROM extensions to support or replace the original graphics capabilities in the provided BASIC.
Is it even feasable to provide ROMs for the 65C02 on the slots (like an address line to trigger its visibility when changing the banks) ?