A few important things to bear in mind here:
- metal cases MUST be earthed (not doing so means you potentially could have a fault that makes the case live). Julien's diagram has this noted, but I wanted to make this very clear.
- use an internal fuse on the mains-side, not just the low-voltage side, on the live wire (not the neutral). 1A fuse would do.
If you are unsure about the wiring for the transformer, switch, fuse etc., i suggest you simply do not attempt it. Just use the wallwart as the designer planned. Any mistake on the mains wiring could electrocute you and/or cause a fire. You also may have live wiring exposed when you have the case open while testing it etc.
If you do attempt it, you can use an insulating "boot" over the mains socket and fuse or use heatshrink sleeving to cover it. Not important for the low voltage side.
Here is a voltage selector switch of the type that could be used to switch the primary from series (240) to parallel (120)
http://uk.farnell.com/schurter/0033-450 ... dp/1211994
Another thing to bear in mind is that putting the mains transformer close to the audio circuits might cause more mains hum pickup - hard to get rid of.
I do have a question about this... if using a metal case but powering using the AC wallwart, which ground point in the circuit should the case be grounded to in order to best reduce the hum?
Stay alive!