The single most common mistake I've seen is not keeping the audio system well isolated from the preamp. This applies not only to layout, but also to circuit design.
In general, in non-microprocessed systems, my experience has been that layout is non-critical, if you use common sense esp. with respect to keeping ground runs short. Until quite recently I did most of my prototyping on perfboards without groundplane, and have never observed any improvement resulting from putting the thing onto a real PC board for production.
Some people like to spread things out, on the assumption that this reduces capacitive coupling. However, I always cram everything together as tight as possible, except sometimes I'll move a circuit block away from another circuit block. Cramming everything together reduces resistive and inductive impedance which can lead to ground loops; and, although it may not be obvious, it reduces some types of capacitive coupling as well.
If the design includes a digital subsystem with variable execution such as a microprocessor, lots of things can go wrong in the way of flicker noise on power supplies, spikes from digital signals hitting op amp outputs and causing them to ring, and capacitive coupling back into the "front end" causing ordinary electrical interference. The problems are somewhat related to those which can happen due to carelessness with audio, and circuit design is involved as well as layout.
--Dave J.
