Firefox recently updated to V57.0.4 on a Windows 7 machine. YouTube videos now play a double soundtrack.
- One soundtrack starts automatically as soon as the video loads, prior to clicking the play button or completion of visual rendering.
- When the play button is clicked, or in some cases when visual rendering is complete but without clicking Play, both the video and audio start.
So there is the normal video/audio plus the duplicate audio track that started on its own some seconds before.
The videos play normally on other browsers on that machine, and on Firefox V56 on other machines. So far, YouTube videos are the only ones found to have this problem.
Note: this is a self-answered question, so I won't include all of the diagnostic details in the question, itself. This is intended to share the solution with others encountering a similar problem.