Sound on specific apps

Yesterday I used Instagram and the sound worked perfectly fine. However when I got on the app, no sound came through. I turned up the volume manually on the side but nothing came through. So I opened Spotify and the sound worked perfectly fine so I thought it was an issue with Instagram. Then I opened Marco Polo and there was no sound either. I can hear notifications, calls, and texts. Why is my phone restricting certain apps from playing any sound?