The next step seems to be for @Inti_Luna and @yanokwa to talk and I'll share a few thoughts in the mean time.
Really good to know about the Adaptive Multi-Rate audio codec (amr) which I was not familiar with. Thanks, @aurdipas. I hope at some point I can explore how it compares to aac in terms of file size and fidelity.
Looking at APK Mirror, it's likely that what @aurdipas used was an early version of the Sony app that we recommended. It's too bad that it has been discontinued. All its APKs are available on APK Mirror. I've successfully downloaded and used the latest but please be aware there is some risk in this approach since APKs may not be as carefully vetted as on the Play Store.
Historically, recording was done through an external app to limit the already large scope of Collect and to let users with external microphones and other specialized needs pick an appropriate tool. It does seem like this has been inconvenient for the common case.
This ties in with @Tino_Kreutzer's rough spec at Create automatic background recording of interview in Collect.