frontend: Mark nested audio sources as previewed

[Warchamp7]

Description

Makes the check for previewed audio sources to be recursive for nested scenes or groups.

Fixes #13297

Motivation and Context

Ensure all audio sources in the previewed scene are properly marked as Preview.

How Has This Been Tested?

Tested with a scene that had grouped audio sources, and a scene with a subscene containing audio sources.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

• Tweak (non-breaking change to improve existing functionality)

Checklist:

• I have read the contributing document.
• My code has been run through clang-format.
• My code follows the project's style guidelines
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.