If it's high end audio I expect you have a spectrum up to a few hundreds of kHz. That's not particularly high in terms of the risk of capacitive coupling between traces, but there is a number of other factors.
- impedance of the traces. A high impedance trace is more susceptible to coupling,
- the energy in the coupling trace. For capacitive coupling the voltage is the relevant factor, for inductive coupling it's the current,
- length of the traces. It's obvious that longer parallel traces have better coupling.
All this means that there's not a single simple formula for the required clearance. For the given frequencies I would choose 20 mils (0.5mm), maybe increase to 100 mils (2.5mm) for higher voltage/current traces next to high impedance ones.
That's actually two separate questions. It's the circuit voltages that determine the clearance requirements, while the current levels determine the width (and thickness) requirements.
Trace Width
Dealing with the latter first, it's the width and thickness of a copper trace on a PCB that determine its cross-sectional area, in the same way that the diameter does for an ordinary wire. The cross-sectional area determines its resistance per unit length, at which point, it's up to you to decide two things:
One or the other of these will be the limiting factor for each trace.
For example, you might have "1 oz." copper on your PCB. This is shorthand notation for "1 ounce of copper per square foot", which translates to a thickness of 1.38 mils, or 0.035 mm. A trace that's 10 mils (0.254 mm) wide, then, has a cross-sectional area of 13.8 mil2 which is roughly equivalent to an AWG38 wire. It will have a resistance of about 0.75 Ω/ft. and the current capacity is on the order of 10s of mA.
To handle higher currents, you might select "2 oz." copper (0.070 mm thick) and use traces that are, say, 100 mils (2.54 mm) wide. This gives you a cross-sectional area of 276 mil2 which is roughly equivalent to an AWG24 wire.
Note that since the traces on a PCB are very flat and wide, they're actually much better at getting rid of heat to the enviroment than the equivalent circuilar wire is. So as far as I2R losses are concerned, you can put a lot more current through a PCB trace — but you still need to pay attention to the temperature rise and the associated thermal management.
Clearance
The required spacing between conductors is determined by the voltage difference between them and the amount of leakage current you can tolerate. Leakage current is primarily associated with surface contamination of the PCB (e.g., residual flux, as well as accumulated dust, moisture, etc.).
One guideline comes from safety testing services such as UL, which requires a creepage distance of 5mm per kilovolt for circuits that are supposed to be "isolated" from each other (material group I, pollution degree 2 from UL840).
Obviously, this guideline gives very small values for low voltages (0.05 mm or 0.002 in. at 10 V), so the limiting factor actually becomes the line/space widths that your PCB fab house is capable of.
Best Answer
Here's what Creepage and clearance means in UL60950: -
Numbers in millimetres.
"Functional" is the minimal that you are aiming at so, 12 thousandths of an inch is nowhere near enough. Across a plain PCB clearance supersedes creepage distance unless you can force slots into the PCB material.