I'm just another Google Nest customer. The volume of a doorbell notification on your phone depends on the volume at which your phone is set. The volume of a "Visitor announcement" (https://support.google.com/googlenest/answer/7672451) depends on the volume at which your Nest Hubs and Nest Minis are set. The volume of your indoor doorbell chime depends on your doorbell chime, and the volume of the doorbell sound on the doorbell itself cannot be set or changed.
