Shaping ground plane on a 4 layer USB Midi controller PCB
I suggest you use ONE plane. All your analog sources are ON THE PCB, right?

Encourage the digital noise to remain in the MCU, by using 100,000 ohm resistors between the MCU and the logic inputs to the analog multiplexors. Adding 100pF caps may be key, as the caps help shunt any high-freq trash to Ground, reducing the trash entering your MUX logic inputs, which would upset the MUX Ground pins etc as the Trash explores ALL possible return paths back to the MCU.

Further encourage the digital noise to remain in the MCU, by establishing a cascade of RCRC filters in the VDD to the MCU. I call this a "local battery", which is tasked with providing the high-speed-charge demands of the MCU. You will still have Ground Loops, but you won't care, because the currents in the Ground Loops will be very small and of very low frequency.

Thusly







simulate this circuit &ndash; Schematic created using CircuitLab

Notice we also use the "local battery" idea, to provide very clean VDD to the analog MUXES. Otherwise the VDD trash gets injected thru the large switch capacitances (FET gate-channel capacitances) into your signals.