I assume you are thinking of a mains isolation transformer for your audio equipment.
Using the 230 to 12 to 230 double transformer will work but if you have to drive a 500W amplifier you will need say a 700W and a 600W transformer to do the job remotely well, probably better than consumer grade.
The popular source for 500W to 1200W isolation transformers for hobby use is to hack some microwave oven transformers. If you have two same (or close model) microwave oven transformers with some hard work you can modify them so that you have two 230V primary windings on one core. This gives you a reasonable isolation transformer (it will likely run a bit hot at full load because they are close to saturation on the cores as they skimp on the turns in the windings) from two scrap microwave ovens.
There are lots of tutorials on how to do this on the net, search for MOT and isolation