This question of sounds in OneSignal push notifications is rather related to the OneSignal SDK instead of the WebViewGold app template in general. That’s why the OneSignal documentation is relevant here:
Follow iOS steps here (“iOS Native”): https://documentation.onesignal.com/docs/customize-notification-sounds#ios
Follow Android steps here (“Android & Amazon Native”): https://documentation.onesignal.com/docs/customize-notification-sounds#android-and-derivatives-like-amazon