Fix: 'Sound' named track is not automatically exported

Description of fix

Put back a refactor version of auto sound channel addition into export.

How to test it

Export any timeline with a track called "Sound" or "sound".

Checklist

Please check if your MR fulfills the following requirements:

• Build was run locally and any changes were pushed
• Lint has passed locally without error
• CHANGELOG.md has been updated
• Branch has been rebased on merge branch

FYI: @alefeve