There is no proof. The evidence supporting these claims seems to be lacking, but whether this is because they haven't been investigated or because they are false is not immediately apparent. There are some reasons to think that specific frequencies or ranges of frequencies could have effects beyond the brain, as is obvious to any deaf person, or any person standing near a speaker at a concert, or any wineglass undergoing this experiment:
Breaking Glass with Sound, or any droplets in this one:
Of interest is the following study which found that sound waves (not 528 Hz specifically) directed at a Chrysanthemum "had no obvious influence on the content of DNA but accelerated the synthesis of RNA and soluble protein. By means of the assay of relation, the content of soluble protein had a very close relationship with that of RNA. This result indicated that some stress-induced genes might be switched on under sound stimulation and the level of transcription increased."
(
Effect of sound wave on the synthesis of nucleic acid and protein in chrysanthemum)
Of more specific interest is an experiment undertaken by John Hutchinson and his partner, Nancy Lazaryan. The couple claim to have successfully treated water from the Gulf Oil Spill by exposing it to 528 Hz. John Hutchinson is considered a dubious source by many mainstream scientists, and his experimental design is not well described in the available articles, but his samples were tested by Robert Naman, president of Analytical Chemical Testing Laboratory and fellow of the American Institute of Chemists: (These are the results:
Page on Pesn). The results indicate that the sample said to have been exposed to 528 Hz contained significantly less oil and grease than the sample said not to have been exposed to 528 Hz. These results indicate A) Poor sampling, B) Falsification, or C) That 528 Hz had an effect.
It doesn't
seem likely that 528 Hz would have such an effect, but the history of science is full of now accepted things that didn't seem likely at first, and the tendency to dismiss any claim on the basis that it doesn't
seem likely, has bogged down science throughout history. Obviously there are times when it makes sense to dismiss "outrageous" claims, when there is a concern about wasting time and funds. Science can be bogged down by investigating things that don't seem worth the effort, too. If this were a claim of a complicated perpetual motion device involving a lot of funding and time and preparation to replicate, it would be understandable to apply Occam's Razor and be done with it, but this is a very easy experiment to reproduce, and the possibility should not be dismissed until it has been attempted.
(Note: Some sources come from the following article, which the author of this answer is not associated with:
The Solfeggio Scale, 528Hz C, Love, Music and How Sound Affects Us)