I just bought that exact case and did my first swim today. It worked great. I used an old phone I didn't care about though. I did do a waterpoof test on the case first at home.

I bought these headphones though: https://www.amazon.com/AGPTEK-Waterproof-Earphones-Swimming-Extension/dp/B075QJPZX1/ref=sr_1_5?keywords=waterproof+headphones+swimming&qid=1576281729&sr=8-5

The reviewers said they worked as good as the expensive ones and they worked great today. I wanted it for audio books, not music. I need to do some fine tuning with fit, but otherwise it went swimmingly.