Forgive me if I'm missing the point here (you did say distribution rather than long-term control so I may well be), but surely it only takes one legitimate user to share the file and everyone can get it for free? Not sure how much use passwords will be in this scenario.
"Watermarking" in the music industry is a closely guarded secret, there are various means of achieving the aim, and once a relatively successful method is found companies are notoriously reluctant to discuss it for obvious reasons. Obviously the technique won't stop people distributing the file, but at least you'll be able to trace who leaked it...
Whether you want to go through the aggro of watermarking would depend on how commercially viable your product is long-term. There are "semi pro" packages available for the purpose of watermarking, but the more sophisticated models will cost you, and probably require you to send the files to them to be watermarked, with only the company knowing where and how to find said mark.
Let me know if WM techniques are of interest, and I can direct you to forums on the 'web where Mastering Engineers can direct you to the relevant firms.
Cheers!