Improve/simplify SoundEffects.cpp example

[vittorioromeo]

Same principle as #3062 and #3071, basically:

• Remove unnecessary layers of abstraction
• Pass dependencies of effects explicitly rather than relying on global state or initialization in constructors
• Avoid unnecessary usage of std::optional for music resources
• Minor readability improvements

Note that I still get segfaults in this example, with or without this PR. See #3079 for context.

[coveralls]

Pull Request Test Coverage Report for Build 9465573843

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.5%) to 54.24%

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Totals
Change from base Build 9463943212:	-1.5%
Covered Lines:	8837
Relevant Lines:	15465

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💛 - Coveralls