I've gotten to the point in my music locker project, MLocker, that I'm ready to be more serious about the mobile considerations. On the desktop, I use it every day to listen to tunes and I don't think I've ever written any software that I've found this useful, not even the forums. It's not something that I can sell, but it's extremely useful to individuals who want to own their music and listen to it anywhere (and can provision the service).
The good news up front is that it runs just fine on Android phones in Chrome, with the screen locked. It doesn't look right, because it's not the most responsive thing yet, but it absolutely works. Building a mobile app would provide two more benefits: Easily storing the files locally, so you can listen offline and not use mobile data, and casting to other devices. But as I've thought more about it, you don't need an app for this.
Remember when Steve Jobs said, "The Web is the app?" Sure, Apple ended up introducing native apps and an entire ecosystem for selling them, but he was absolutely right. Games aside, there are few things you need an actual app for. The current legal battle between Epic Games and Apple is indeed a crisis of Epic's own making, but it importantly calls out the fact that it's the only channel you have to distribute software for iThings, and that sucks. Android is only a little better, but you can in fact install another app store. I think it's lame that you need to give Apple a hundred bucks to sell an app, and then they keep 30% of what you charge.
Anyway, I started to look into writing a few apps since I could easily reuse much of the code from the web front end for MLocker. Xamarin could handily do this, but I realize now that I kind of hate XAML. I haven't used it in years (not since before Silverlight died), and I remember now that it's totally over-engineered. On the plus side, there's a side project team working on Blazor-for-mobile, but it's not very well baked. The idea of doing this brings me no joy at all. That's when I started to be real and think about how I really don't have to.
So first problem, casting, that's actually already solved. You can use Google Home to cast whatever audio your phone makes, regardless of app. You can also Bluetooth to an Alexa, so that's solved.
The second problem, the off-line scenario, as it turns out is not that hard to solve. Progressive web apps don't get enough love, but they're a well-defined standard that is implemented everywhere and it works. You can keep stuff on the phone and even make it "app like" without a lot of friction. I don't know why I didn't think of that before, it's something I've looked into enough to see that it's not a big deal.