I don't think having a stereo built into the hot tub is a bad idea. People will say it is just one more thing that can go wrong with the tub. If you have that feeling about it then why do we have power windows, power seats, in cars? We don't need a lot of what we buy. For some people it is nice to be able to have the choice to buy them.
I remember when I got my first vehicle with heated and cooling seats. It was an option that I though was a stupid idea till I got it. Now ever car I have has it. We as Americans what more and more " extra things" in everything we buy.
It is a choice. If you like it, want it, can afford it, buy it.
You misunderstand why I'm not a fan of stereos in spas. I'm not saying the idea of music in a spa is stupid, to each his own. I'm not someone who responds with something like "Relax in your spa, you don't need music or a TV...". I don't try to regulate what people want, I respond more based on how I think it'll work out for them
Power windows, seats etc. in cars perform well over time. I'm speaking from experience when saying stereos in spas have a very poor history the past decade, to put it mildly, due to the overall environment they reside in. There is a reason they always come with a 1 year warranty as many have learned the hard way.
Again, if someone wants one fine but I'm just relaying what I think about how it'll hold up. Maybe that wil change in time but until it does I'd skip it