Every smartphone comes with an MP3 player, it's a pretty standard feature and has been for more than a decade. So there is no need to buy a separate MP3 player anymore unless of course it has some defining features.
Sony has an MP3 player in its Walkman series which has one very major defining feature - it's waterproof. And what better way to sell the product than to put it in a bottle of water. With it being targeted at athletes, they'll of course get a water bottle along with it.
To push it even further, the bottles will be selling in vending machines, alongside other drinks. With most gyms and sports centres having vending machines, it's a brilliant place for the product.