There are two reasons why fraud doesn't typically apply with audiophile nonsense. Fraud requires mens rea - that is, intention to defraud, which requires that the seller not believe his own bullshit. There are definitely examples of genuine scammers who don't believe their own BS in the audiophile world, but more often than not, the people selling this BS actually believe at least a substantial portion of it themselves.
The other issue is separating what's known in the law around advertising and consumer protection as "advertising puffery" from fraudulent claims. If I sell you a $10,000 silver cable with a bunch of fancy insulation and say "it'll make your stereo sound better", and you actually *receive* a silver cable with a bunch of fancy insulation, there's a fairly strong argument that the asserting "it'll make your stereo sound better!" is advertising puffery. Generally when you're selling products, you're allowed to say "this product is better than that product!" when there's an element of subjective preference involved.
Where it would be actual fraud is if you sold something as a silver cable with a bunch of fancy insulation and it was just a bog-standard aluminum wire wrapped in some fancy heat shrink.