Kindly follow the below-mentioned steps to resolve your concern:
Method 1:
realme UI 3.0 & above versions: Long press the app icon > App info > Manage notifications > Turn off Ringtone
realme UI 2.0: Not supported
realme UI R & Go & T & S & U Edition: Not supported
Method 2:
realme UI 3.0 & above versions: : Settings > Sound & vibration > Notification > Turn off the Notification volume
realme UI 2.0: Settings > Sound & vibration > Notification sound > None
realme UI R & Go & S & T & U Edition: Settings > Sound & vibration > Turn off Notification volume