Almost none. I ran unshielded tonearm cable in my home system for some years. You could crank up the volume and no hum or buzz from the cable, even if grasping it or moving it around. You do still have to ground the tone arm and it works best if that wiring travels with the signal wires. One advantage here is this allows for much lower capacitance in the cable.
@atmasphere Wire to shield capacitance should play role only if either output or input circuit is ground referenced. I thought it was the advantage of fully balanced, not ground referenced (floating) configuration. There is still wire to wire capacitance (increased by twisting), but there is no wire to shield capacitance, making even less expensive cables to sound better. I agree that shield does not improve much, since twisting wires makes them very immune to electromagnetic or capacitive pickup, but it should not cause buzzing. What lusima31 described sounds like ground loop. XLR cable shield is grounded at both ends, but shells shouldn't be, since it might create such loop. I've read that in recording studios they often have problems with that and fix it by cutting shield at one end.
