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.
With this implementation that won't work, as `set_looped()` requires a different internal variable to be toggled on before calling `startSound()`.