At a guess, I would say that the same rules that apply to other assets on the DLS would also apply to sounds and engine specs - check the config.txt files for any license instructions and, even if the creator has given it away as total freeware, acknowledge them in your config.txt files as a common courtesy.
But I am not sure what you mean by "public release"? Does it mean DLS only, third party Trainz web sites or to be released for non-Trainz uses?