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This can be done if you're comfortable with using a userChrome.css file. (If you haven't done so before, here's an introduction to how to do so: https://www.userchrome.org/how-create-userchrome-css.html)

The code to bring back the favicon is:

  .tab-content .tab-icon-image {
    opacity: 1.0 !important;
  }

If you want to hide the speaker icon (it probably looks kind of messy when overlaid on the favicon) then:

  tab[soundplaying] .tab-icon-overlay {
    display: none !important;
  }

Related: https://support.mozilla.org/en-US/questions/1339041 (this is how I stopped the 'mute' icon from interfering with the favicon).

This is affected by the Density settings you can select on the Customize page. In Compact mode you only see the sound playing icon and not the website favicon. In Normal mode you see the favicon by default and the sound playing icon on hover, there is a secondary line that shows the 'playing' or 'muted' state since the icon is hidden by default.

Well thank you @cor-el. It's true. After switching from Compact mode to Normal (adjusting the settings for the toolbars), the speaker symbol will change from favicon when you drag the mouse over the Youtube TAB. However, I would like to use Compact mode and at the same time see the favicon + speaker icon (or a similiar favicon for "playling" status) as before. This is because Normal mode shows larger TABs than Compact mode.

So... for my purpose, the first CSS code from @pg_78 helped. Thanks ! In this case, I see a favicon according to Youtube when playing a Youtube video - it's a symbol of a red speaker playing - it's probably the original favicon (sent from the Youtube website). So, I know that there is Youtube website on this TAB and at the same time that some video is being played there.

Thanks guys... thank you both for the quick reply.