Where can I find build in sounds in fcpx I a editing a video and I want to add some sound effects ie by some places add some small Bits of audio. Is there built in audio files in fcpx like in iMovie? Or do I need a plug-in? And if that is the case, what plugin?

Thank you
Make sure you do “Download Additional Content” under the Final Cut Pro menu. Download and install if you haven’t. 

Left top corner next to the Library Sidebar icon is the Photo and Audio Sidebar Icon. When you in this sidebar you can find “Sound Effects” with lots of sounds you can use for free.