How to calculate typical signal trace clearance?
The necessary clearance will vary between different traces, depending on the factors Steven mentions.
With Audio design there is often a high impedance, small signal front end, and a low impedance large signal output. For instance in a mic preamp you may have a gain of over 60dB with e.g. >1k ohm input impedance and &lt;50 ohm output. Any traces from the two need to be kept well apart.   The first PCB I made for a mic preamp turned into an excellent oscillator as I didn't pay enough attention to this.
Of course this applies to intermediate signals too but the risk/difference is likely the worst at input/output.
Also, if you use in/out transformers then be sure to keep them apart, as the same thing can happen with coupling of the flux. Input transformers are normally shielded to protect from things like this, but unless they have very good shielding and/or aligned just right then placing right next to each other is asking for trouble.  

For a good book on Audio design I found Douglas Self's "Small Signal Audio Design" and "Power Amplifier Design Handbook" to be excellent. For PCB design in general often the books dealing with digital design (e.g. "High Speed Digital Design" by Johnson and Graham - old but still worth a look) often contain very useful information, especially about trace-trace coupling, EMI, planes, etc.