Add audio in partial_movie_files, not at the end

Nobody has claimed this yet.

## Description of proposed feature

Currently, ManimCE seems to add audio to video files only at partial files combinations.

I have wondering if adding the audio on partial movie files would be possible instead.

I guess that this could mean some important changes in the codebase, but that would all users using partial movie files or sections, which are currently without sound.

## How can the new feature be used?

I maintain Manim Slides, and that would be great to have audio included in slides (https://github.com/jeertmans/manim-slides/issues/258). An easy solution (but maybe not so easy for ManimCE's maintainers) would be to have the audio contained in each partial movie file.

The issue names no files or tests; start by tracing how partial movie files are rendered and where audio is currently added during file combination. Check the existing partial-file and section workflows first. Done means each partial movie file contains its corresponding audio while existing combinations continue to work.

Written by the indexing model from the issue text.