Nitric Acid is how to test for gold - I'm not aware of any alternatives. If it's a darker gold it's probably brass (or brass plated steel). Manufacturors will not use gold unless it improves the quality of connections, so things like posts and corners will almost certainly be a cheaper metal.
Speaker magnets are only magnetic on each face, since these are the poles. The edge is halfway between two magnetic fields so things rarely stick to them dead on.
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