What's wrong with this simple PCB design? (midi synth)
The 6N137 datasheet says:


  A 0.1 μF bypass capacitor must be connected between pins 5 and 8.


This is required not only to prevent the 6N137 itself from oscillating, but also protects the rest of the circuit from dips in the power supply.

You have an inductor in front of the ATmega's digital power supply, but it might be a better idea to move/copy it to protect the analog part of your circuit from any of the digital parts.