Please make sure that the sound on your device is turned on and the volume turned up.
See if the sound is working on other apps, e.g. Facebook, YouTube, Instagram.
If it is working on other apps, then it's best then to delete/uninstall the Evolutioner app and reinstall it from the app store.
To do that, tap and hold on the app icon and select Uninstall.
Once uninstalled, go the app store and reinstall again.
If that does not work, please see if it works when using the Web App.
Just let us know if you're still having issues.