core: Stop sounds attached to an unloaded root movie

[Toad06]

... instead of doing this for each unloaded display object.

Fixes #18327.
Fixes #18181.
Fixes #18057.
Fixes #17718.
Fixes #17395.
This should fix #17059 as well... but I'm not sure where the issues are supposed to be. 😅

I also checked against the files mentioned in #17016 and can confirm that they continue to stop sounds as expected. However, I was unable to test #16947 due to the web server being down. For #963, I tested it on FlashArch, since the file appears to be no longer accessible on the official site.

Super Monkey Ball Mini is an interesting test case, as it experienced both issues in sequence:

• Before #17016, the music from the title screen continued playing when entering a level instead of stopping.
• After #17016, while the title screen music did stop correctly, some sound effects unexpectedly stopped (for example, when clicking on a level index) or failed to play altogether (such as the "Go" sound when starting a level).

[torokati44]

Thank you! 🚀

[Toad06]

Thanks for the review and merge!