Downloading sound for my 2D animation

At this point i needed sound to add into my animation, from watching other peoples work on Youtube i learned that all the did was add a soundtrack into the back, however the more advanced animation that were done professionally had proper sound, this meant the sound would flow with the animation. i didn't want to just add a soundtrack in i wanted the sound to flow with the animation, the reason i chose to do this was because it would ake my animation look more interesting and professional, and for this reason i downloaded sounds of buses, cars, and car cashes. this was all downloaded of youtube and in a MP3 format as this is the most common sound format and i knew it would be compatible to most editing softwares.