Disclaimer: Unfortunately, I could not find any scientific sources or studies on this, so my answer is purely speculative and not necessarily correct - merely an opinion.
I also use these sounds to goad my cat sleep with me. I can see that he gets unsettled, as there are dangerous territorial cats around that he cannot see, but it usually takes five ten minutes for him to relax.
From a medical standpoint, you aren't actually harming your cat. If your cat relaxes after 5-10 minutes and doesn't seem to be stressed-out beyond some tension while the sound is playing, I can't see any serious long-term side effects other than being a bit more territorial or anxious with other cats.
So, how cruel is this practice?
From an ethical standpoint, well... it is manipulating an innocent animal to follow your will/bidding, but ethics are tough.
Here's what you should ask yourself, questions taken from here: (I rank what your response would be to each question out of 4, meaning 4/4 is good and 1/4 is not)
Does your decision conflict with any of the core ethical values?
It does conflict with some... for example, you are deceiving someone, and not treating someone how they would want to be treated. 2/4
Think of someone whose moral judgment you respect. What would that person do?
This is a question that only you can answer. However, if I asked myself, "what would a vet do here", their response would probably be to avoid deceiving your pet as much as possible. I will not rank this as you need to do this one yourself. N/A
How will your decision affect others?
Your decision is ultimately fooling your cat for your benefit, a form of trickery. Is this positive or negative for your cat? Negative. 1/4
Ask yourself: Are my actions legal?
To my knowledge, this is not illegal anywhere. 4/4
Are there regulations, rules, or policies that restrict your choices/ actions?
To my knowledge, there are not any policies, however, if this strategy is used too often, it could qualify for animal abuse. It doesn't seem too frequent, though. 3/4
Would your decision be perceived as unethical?
Ask a friend for advice. What do they think? N/A
How would your decision look if it were reported on the news or in another public forum?
Ironically enough, this website is similar to a public forum, but the question assumes a highly popular/visited forum or news site. It wouldn't be a positive story, but I doubt that you would be getting death threats. 2/4
What would a reasonable person do? How would they perceive your decision?
Again, ask a vet for their honest opinion. N/A
Would you be proud of your choice if your child were to find out? Would you want them to make the same choice?
Would you? I don't think you would be ashamed of yourself, but I think you would discourage them from doing the same. 2/4
Could you rationally and honestly defend your decision?
You probably could, with the defense that it is not medically harming your cat. You might not win, but you could put up a fight. 3/4
Will you sleep soundly tonight?
I think you will, especially with your cat snuggling with you! (that was a joke, please don't take that as a recommendation to use noises to influence your cat) 4/4
There were 32 possible points you could earn, and you earned 21 of them. Three questions were N/A and you need to asses them yourself. Overall, 21/32 isn't bad, it's more than one-half positive, meaning it is leaning toward ethical.
However, these were just guidance questions, and ultimately, this is up to you and what your vet thinks.