I suspect there exist filters in the browser APIs to adjust play back timing and rate. I have not attempted to modify the media in that way myself but I did see API filters to alter stero panning, tonal quality, wave shape, biquad, delay, and more.

I just wanted to listen to music I already have played from my phone while I do dishes.

In my case I just wanted to have a media player that could play MP3s and feature a playlist dynamically populated from either the local device file system or, more ideally, from a shared network location. Restricted. There were some media apps that kind of, in a really shitty way, figured this out. Most of those apps, even when shitty, cost money from the app store.

Fuck that stupidity.

I wrote my own solution in JavaScript that executes in the browser and it does almost everything I want. The playlist data must be statically written into the browser page and cannot be dynamically populated from any file system because Apple disables that part of the browser's FileSystem API in browsers on iOS, though it executes correctly on desktop Safari. I then host the page from a shared network location so that I can access the same content in the same way on any device with access to the network. I adapted this home grown media player to execute video as well, but it can only play videos from codecs/containers already supported in the browser.

OPUS is a descendant of CELT (which never caught on, for reasons) which is an open/unhindered by patents low latency lossy codec suitable for VoIP and live streaming. It's relatively recent, but also, those use cases are more popular recently in the browser.

And to go even more extreme, some applications really care about bit-exactness in their decoders, which you don't get by relying on other vendors.