What phone still has a 3.5mm headphone jack? Sincere question, I'd like to buy one.
Samsung Galaxy A Series (A15 / A15 5G, A14 / A14 5G, A25) all have headphone jacks. These are their "lower-end" models, though. The higher-end ones don't. And of course, iPhones haven't for a long time, too. Alternatively, for other phones with just a USB-C port, you could get a USB-C to headphone jack adapter.
edit to add: if Slate is successful, I wouldn't be surprised if a decently sized ecosystem pops up around easily installed custom sound systems and the tablets (possibly with headphone jacks!) to control them.