[Annile Brown]

I think you should split the audio files to several parts.
Select the specific part which you want to slow down, and edit it with audio editor.
With regard to the normal part, just keep it as original.
I searched a tutorial in Google, hope it helps.
https://www.apowersoft.com/slow-down-audio.html

[Annile Brown]

In order to complete this task smoothly, you can use an all-in-one video recorder. It features both downloading and converting functions. If you only want to make some small changes on downloaded video files, the inlaid video editor of this app will be your helper.For advanced video editting operations, I recommend Windows Movie Marker and Camstudio.