Now that someone has mentioned tar:
In the 1990s, I worked for a company which manufactured, among other things, traditional (magnetic) fluorescent light ballasts.
Because of their embedded air gaps, those transformers would buzz like crazy.
Those were enclosed in sand-filled tar. The best sound deadening compound you can find.
That was applied at a very high temperature with specialized equipment. Because the high temperature and that the tar will strongly adhere to the skin, which would cause extensive third-degree burns, THIS IS VERY DANGEROUS to do it manually.