Sometimes if you are switching to other apps that also use sound, your device can get confused and stop playing certain kinds of sound. Force quitting and relaunching the app should resolve this.

If you are using an iPad or iPhone, can you please check the switch on the side of your device near the volume controls? In My Singing Monsters, this switch serves as a mute switch. If you do not have the switch on the side of your device, then the switch may be displayed as a bell icon in your control center.