Allow use of FlxSound if FLX_NO_SOUND_SYSTEM, make loop a public property

On a related note, it would be nice if flixel sounds weren't included when `FLX_NO_SOUND_SYSTEM` was enabled, but I don't know how to make that work with the current setup.
What happens when you set `looped = false` after it has stopped playing? I guess it doesn't start playing again?

What to do you think the expected behaviour there is?