[encom]

By MP4 I assume you mean AAC in MP4. I'm a bit confused by this. What emits AAC that then needs transcoding to Opus? Not that transcoding losses really matter for this application, but the pipeline is not clear to me?

Very interesting project, by the way.

[smalltorch]

So this is only for termux compatibility. On a standard distro it skips this step entirely and goes straight from raw pcm to opus.

On termux 'termux-microphone-record' is a wrapper around androids 'mediarecorder'. It doesn't support raw pcm output. It records AAC in m4a wrapper and then the extra ffmpeg package converts this to rawpcm so it can follow the same pipeline.