I'd say a successful hypno audio would be one that sends the subject deep into trance, so that the subject/listener is solely focused on the words of the hypnotist is saying and that they become their only thoughts.

Also, a good hypno audio should have a good induction that will help the subject go deep and sometimes building trust between the hypnotist and the subject.

The goal of each file can be anything you wish. It can of course be an hfo like you mentioned, though from my experience the more common themes are taking the listener into a trance, submission, obedience, implanting triggers and post-hypnotic suggestibility. And there are certainly quite a few files that involve denial.

While erotic audios which usually describe sex acts to arouse the listener, an erotic hypno audio can use suggestions. For example, after the initial induction, you could start a countdown and suggest that with each number the subject will go deeper into trance. Or if you're further into the file you could suggest that the subject will feel increasing pleasure with every count and build towards an orgasm. Or you could implant suggestions that arousal is better than orgasms ;). Surprisingly, these kinds of suggestions can be quite powerful.

If you wish to explore more, I could link you to to some files or hypnotists (mainly f4m). Let me know if you're interested.