At least in the UK we have the Advertising Standards Authority
https://www.asa.org.uk/ . Based on a number of cases against audio companies you can:
1. Provide measurements, for instance of RF attenuation of a product (say a power regenerator), or harmonic distortion of a power amp, or whatever.
2. What you cannot claim is that it improves the sound quality. In other words you cannot claim that a measurable product feature will produce a subjective improvement.
3. You *can* say that in your own listening tests you hear x, y and z, but you must state that this is your opinion, and not a guarantee that a customer will hear the same thing.
4. If you cannot make any measurements, or like AudioQuest make an optical SPDIF connection by bundling umpteen fibers in a single connection, you can say *why* this, in your opinion, is an improvement on earlier techniques (and often expose yourself as a charlatan in the process) - but you cannot claim that a purchaser will hear any difference or improvement.
If any company, including audio companies, contravene a CAP code, they have to cease and desist advertising, or carry out corrective actions to their advertising to comply. To trigger this all you have to do is fill in the form on the ASA site and provide evidence.
https://www.asa.org.uk/codes-and-rul...cast-code.html
Craig