sound examples: Organize files in directories

More examples will be added, so it's better to be organized.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D45968

(cherry picked from commit cb00491fbc51751fe166056cc69e23beeb26fee4)