libraries_get_path('jplayer') returns empty path.

libraries_get_path('jplayer') returns empty string, therefore _jplayer_get_js() will always look in wrong folder for JS files (it will only work if jplayer JS is in root folder!).
Reason for that is I think in hook_library() where you made circular dependency by calling:

$file = _jplayer_get_js();

which then calls libraries_get_path('jplayer') !!

hook_library should have clearly defined path instead of calling on itself to get path.