sounds: Add PC speaker sounds by GhostlyDeath

These are some initial PC speaker sounds that cover most of the common
sound effects, made by GhostlyDeath. Some discussion can be seen in
the thread on Doomworld:

http://www.doomworld.com/vb/freedoom/65535-wip-pc-speaker-sounds/

Signed-off-by: Simon Howard <fraggle@gmail.com>