According to Google's best practices for interstitial ads:-
https://developers.google.com/mobile-ads-sdk/docs/admob/android/interstitial
Remember to pause the action when displaying an interstitial. There
are a number of different types of interstitial ads: text, image,
video, and more. It's important to make sure that when your
application displays an interstitial, it also suspends its use of some
resources in order to allow the ad to take advantage of them. For
example, when you make the call to display an interstitial, be sure to
pause any audio output being produced by your app. You can resume
playing sounds in the onAdClosed event handler, which will be invoked
when the user has finished interacting with the ad. In addition,
consider temporarily halting any intense computation tasks (such as a
game loop) while the ad is being displayed. This will make sure the
user doesn't experience janky graphics or stuttered video.
Summary: Sound is paused so that user can view ad without any issue. While you can definitely play the sound in background , I would advice not to do that as it would interfere with ads.