Yes, it is true. Even if you lower the volume with the volume buttons or the control center, connect wired / wireless earphones, or set the switch on the side to the position where you can see the orange color (mute mode), sound may come out from the speaker built into the iPhone. There is. Use caution when you are in a place where you should not make noises, or a place where you will be disturbed if you make a sound accidentally.
Regardless of the sound setting, the alarm / timer sound and camera shutter sound are output from the built-in speaker. Even if you have wired / wireless earphones connected, the sound will come out of the built-in speakers regardless of that. The volume of the alarm / timer sound can be adjusted, but it cannot be completely muted.
When in mute mode, the ringtone and notification sound will not be heard from the built-in speaker. Most apps, such as games, also have their built-in speakers disabled and no screenshots or other sounds. However, there is an exception for apps that play music / videos (those that can be played / stopped using the control panel at the top right of the Control Center).
If the sound is still bad from the iPhone, make sure the switch on the side is in the mute mode where you can see the orange color, then make sure that the alarm / timer is not set, Make sure you do not shoot and do not launch music / video playback apps.
This is almost perfect, but there is a possibility that you will receive an emergency bulletin that notifies you of dangers such as earthquakes and tsunamis. When the emergency alert is received, the alarm sounds at the maximum volume regardless of whether it is in the mute mode or not, so if you want to be absolutely sure of silence, go to "Settings" → "Emergency Alert" switch on the "Notification" screen Consider turning off.