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The midi specification mentions optional capacitors between connector shield and ground for a midi input:
Neither the type of capacitor nor the voltage rating is specified. I would think that the voltage on this capacitor could be quite high if both the sending appliance and the receiving appliance grounds are "floating". What kind of capacitor would be appopriate here?
The left side of the drawing says (twice!) 0.1 uF typical. I expect a 50 or 100 volt ceramic cap would be fine.
The only purpose of the MIDI isolation is to prevent ground loops. In practice, there are not any large voltage difference. (Several cheap USB/MIDI interfaces do not have any isolation and connect the grounds together.)
These capacitors are intended to short high-frequency noise to ground. Their voltage rating depends on the induced noise voltage, i.e., it does not need to be large; even 6.3 V would be OK. You do no need the self-healing properties of film caps, so ceramic is fine. And the exact capacitance does not really matter, so you can get away with a cheap dielectric like X5R.
Just use the same ceramic caps that you're using for decoupling elsewhere.
