I want to add game sounds like game over, collision, extra points etc. Is there any free sites to download from?
This is a good site.
As @Harinezumi stated, a lot of these sites infringe copyright laws. There are some free sites online, you can find a list there. But even if you find some free (and legal) sites, you must be careful with the license, and be sure to credit the authors.
You can try youtube mp3 converter for downloading free audio files in just a few clicks.
Tubidy3.Mobi will serve you on music. Tubidy is a kind of music search engine.
Well i haven’t use that much audio in unity but i do know you can use youtube to download audio file. to do that find the youtube video that you want to use for your project, than coby the videos link and then open youtube converter mp3.(but make sure you are using mp3 not 4 if you want to use audio not video) Paste the youtube video link and wola free audio.
Warning: i think this will count as pirate.
(note mp4 is for yt coby of video it self.)