Hi all, newcomer to this forum but not to the Yi cameras and app.
Because I use one of my cameras to monitor my birds, I sometimes switch off sound alert for obvious reasons 
. However, the sound alert option has disappeared. I still have motion sensor. Have I missed something 
Welcome to the forums!
Since the YI IoT app is designed for cameras manufactured by other companies, the available options may vary depending on the camera’s chipset.
When you say ‘Sound Alert’ are you referring to the Alarm Drip option under Smart Detection settings?
If you haven’t already, try to delete the camera from YI IoT app, Reset it, and then pair it back to the app. Ensure you are using the supported WIFI network band when pairing the camera to avoid issues during the pairing process. If you’re still having issues, please contact the support team by sending them an email at Support@yitechnology.com
Thank you