The appeal and strength of the iPhone is that you can understand how to use it without reading the manual. However, there are times when you think you know it but you don't understand it correctly. In this section, we will explain in an easy-to-understand manner "why the iPhone can't be heard anymore". This time, I will answer the question, "It should have been muted, but as soon as I connected the Bluetooth earphones, it became louder !?".
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I should have pressed the volume adjustment button (down) on the side of the iPhone many times to mute it, but when I switched to Bluetooth earphones, it was loud rather than mute? As soon as I switched to Bluetooth earphones, the volume changed drastically. So you would be surprised. However, such a phenomenon is quite possible on the iPhone.
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— Johnna Sun Oct 30 17:47:47 +0000 2016
This is because the audio output destination is handled independently on the iPhone. Devices that can be specified as audio output destinations, such as built-in speakers, Bluetooth earphones, Bluetooth speakers, AirPlay audio devices, AirPods and Apple TVs, have individual audio setting information and separate volume management. Even if the built-in speaker is muted, if you switch to another audio device, that volume (the volume when it was connected last time) will be applied, so it is no wonder that the volume becomes loud in an instant.
In order not to be surprised at the sudden loud volume, it is better to pause the playback of music or video when switching the audio output destination. For example, if you turn on the power of the paired Bluetooth earphone while playing music, you will hear music suddenly at the volume you used last time as soon as the connection is completed.
You can visually check the volume change in an instant with the slider of the audio control in the control center. Let's change the output destination while connecting an audio device other than the built-in speaker such as Bluetooth earphones. Seeing is believing, and you should be able to quickly understand what is happening when you switch the audio device you are outputting to.