The engine suddenly 'roared (back) to life'. [=it made the loud sound of an engine and began running well]
The audience 'roared' with laughter. [=the audience laughed loudly for a long time]
The fans were 'roaring' [=yelling and cheering loudly] after their team scored.
(..) -Yes, sir. I'm sorry. No, sir. For his final appearance of the day, the king of New York City. Alex the lion. Roar! Thank you. Thank you very much. You guys are great. You're a great crowd. Give yourselves a hand. Thank you. Thank you. (..)2005 Madagascar
(..) Show them the cat. Who's the cat? ...the king of New York City. -Alex the lion. -Alex the lion. It's showtime. Roar! Gather around, people. Big show about to start. Check out the zebra taking care of biz. That's right. Just smile and wave, boys. (..)2005 Madagascar