sound examples: Extend and clean up

- Simplify directory and file structure.
- Clean up and improve code. Add more comments.
- Add polling examples.

MFC after: 1 week
Reviewed by: christos
Differen

sound examples: Extend and clean up

- Simplify directory and file structure.
- Clean up and improve code. Add more comments.
- Add polling examples.

MFC after: 1 week
Reviewed by: christos
Differential Revision: https://reviews.freebsd.org/D53353

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