When calling Apple support, it is a mixed bag, depending on to whom you get to speak. If you are not having much success then hang up and call back. Also, the internet has more information than Apple support does, or is wiling to divulge, the tricky part is finding it. Before continuing, please note that, Headphone Accommodations may have an effect on the volume while using headphones. Currently, the iPhone appears to have 5 different categories of volume: Ringtone & Alerts, Media, Siri, Phone Headset, Phone Speaker. These can be confusing because, on the surface, there appears to be some overlap. To clear this up, a breakdown follows:
Ringtone & Alerts: Controls the volume of: Phone ringer, Alarm sounds, and Alerts, including any notifications/alerts made by Siri. The control for this is in: Settings>Sound & Haptics, also, available in the Control Center, but only when these sounds are actively being played (The icon displayed in the bottom of the control when active varies between the Speaker & Phone Handset depending on the Alert/Notification type.).
Media: Controls the volume of Media, such as music, movies, etc. The control for this is in the Control Center, as the default, i.e. when silent and also when media is being played (The Speaker icon is displayed in the bottom of the control, but is not exclusive to this control.).
Siri: Controls the volume of Siri, exclusive of the aforementioned, Siri announced Notifications/Alerts. The control for this is ephemeral, i.e. only appears while Siri is active and is in the Control Center (The Speaker icon is displayed in the bottom of the control when active, but is not exclusive to this control. This control can only be used when Siri is speaking but inactive, i.e. when speaking via a Shortcut command, otherwise one must use the physical volume buttons on the phone to change the volume, while she is speaking.).
Phone Headset: Controls the volume when using the Phone against your ear or with earbuds/headphones during a call. (The Phone Handset icon is displayed in the bottom of the control when active, but is not exclusive to this control.).
Phone Speaker: Controls the volume when using the Speaker Phone option during a call. (The Phone Handset icon is displayed in the bottom of the control when active, but is not exclusive to this control.).
The volume may be controlled via the volume buttons on the phone as well, but only during the same conditions that they are also available in the Control Center. Some exception: During Siri Notifications/Alerts, where they cancel the alert instead. If you have allow buttons to control Ringtones & Alerts disabled.
Conclusion: Change the volume for Ringtone & Alerts to control the volume of Siri Notifications.
If you need any clarification please contact me by reconstructing my email address found at the end of this message since I will not be monitoring this post.
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