Some users are reporting an issue where the iPhone earpiece and built-in speaker stop delivering sound. Generally the issue occurs after the Apple stereo headset or another pair of headphones has been connected then disconnected. It appears that the iPhone, in some instances, does not recognize that the headset has been disconnected, and does not automatically return to pushing audio through the earpiece speaker (for phone functions) and built-in speaker for keyboard tap noises, email alerts, etc.
In some cases, the problem can be resolved by simply plugging and unplugging the headset for headphones several times. This can trigger the iPhone to correctly recognize that no external headset is attached.
In other cases, a full hard reset can prove effective. Turn the iPhone off completely, by pressing and holding the Sleep/Wake button (on top of the device) for a few seconds then slide the red slider. Turn it back on by holding the Sleep/Wake button until the Apple logo appears.
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