There are some useful comments I got from Ultracad some time ago, when I asked about trace width and filling unused copper areas:
1. Does trace width matter?
2. Does copper fill between traces matter?
Generally speaking, almost nothing matters with audio circuits! This is a
slight oversimplification, of course. But there are no high-speed issues
with audio. So trace width doesn't matter. Trace width only matters if you
are using a high enough current that you need the current carrying capacity
of a wider trace.
Copper fills can cause noise coupling problems in high-speed circuits. I've
never seen a slow (audio) circuit where it mattered.
What does matter with audio circuits is power system stability --- power
supplies must be well decoupled so no power supply noise (particularly 50
or 60 cycle hum) can get into the audio circuit. If there is a very
sensitive front end (like a very sensitive pre-amplifier circuit) you might
need to keep that separated from the other traces (and fills) to minimize
external noise from coupling into that part of the circuit. Otherwise,
audio circuits are pretty forgiving.
In the case of line-level audio circuits, as those involving chip or discrete amplifiers seem to fit with, we have to be careful on decoupling power inputs.
your affirmations are nonelementary but the answers can be simple.
- The trackwidth is a very recurrent question among the DIYs, for the power amp build. In some link find those programs to calculate a "adeguate" width. Even the professionals use this programs.
- Audio matter and signal (LF); it is true that the work to HF is a lot of more criticize, but bring the HF rules in audio are able only increase the performances of the PCB, never decrease it.
On the track of the audio pcbs not there are only signals from PSU, but all the intermodulations and harmonic cause from the active comp. work current. If the signal has a little current ( goes RIIA preamp ), doesn't count, because the circuit is a lot of more sensible ( and amplifies more the disturb).
The only difference from the HF pcbs VS audio PCBs that be necessary use a "virtualGND" much more accurate. Naturally the PCB draw is a work much gives a complex, full of compromises, and the experience of the draftsman is fundamental...