You can do it manually in unity only but don't bother, there are great assets for sound. I'm using Master Audio AAA which is a paid solution, but if you want to go free Sound Manager Pro 3 is the way to go. It's got Playmaker support too.
Basically what you need to do is limit the number of voices for the explosion sound. What are voices? If you set the number of voices to 1, the sound won't play if the same sound is already playing. Limiting voices to 1 if you have a lot of explosion can be boring, but listening to a cacophony od same sounds triggered at the same time can also be tedious, so i propose you make a couple of different explosion sounds that will play randomly and experiment with number of voices until you get the best result. Also, play around with sidechain compression (popularly called ducking) to bring out the more important sounds up front.