@jonnylangefeld 's solution initially worked for me, thanks for that.
However, it has a bug when in a progressive web app (PWA) context on IOS Safari. It initially works, but when putting the app in the background and back in the foreground it no longer works (despite reinitialising anything that could potentially be reinitialised). The recording blob is empty. I would not be surprised if there were similar issues in a webview as well thought that’s just a guess.
However, the solution detailed by @neet.kes , aka converting it to an mp3 on the device directly, using mic-recorder-to-mp3 for instance works.
To be clear I think that’s an issue with the polyfill potentially, not with your specfic implementation.