Article in English: If you can hear sound from your headphones after resetting the phone, there might be debris in the headphone port.  This debris might be sending a false signal to the phone that the headphones are plugged in, thus preventing sound from playing from the speakers.  Disconnect your headphones from your iPhone prior to cleaning the headphone port. Remove the cotton from one end of the cotton swab by pinching one end with your thumb and forefinger, then pulling your hands apart to remove the balled cotton.  Once removed, discard the cotton.  Pinch the same end again, loosely this time.  Roll the cotton swab along its axis to spool some of the loose cotton upon itself.  Put the cotton swab in the headphone jack.  Gently guide the narrow end of the cotton swab into the headphone jack.  Turn the cotton swab around a few times, then remove it.  Test the speakers to see if they work.  Scrubbing the headphone jack with a cotton swab is the easiest and most common way to clean the headphone port. Do not dampen the end of the cotton swab with water or rubbing alcohol.  This could damage your iPhone. Lay the phone on a flat surface.  Place the phone in such a position that its headphone port faces you.  Aim the canned air nozzle at the headphone port from the distance recommended by the canned air’s directions label.  Squeeze briefly, then release the handle.  Canned oxygen is a common tool used to clean PC parts, and you should be able to buy some at your local computer or electronics shop. If these steps don't fix the headphone jack on your iPhone, check inside the jack to make sure there are no foreign objects lodged there. Stuck objects may need to be carefully removed using a long, skinny tool like a paperclip or straw.

Summary in Vietnamese:
Cắm tai nghe. Sử dụng tăm bông. Dùng bình xịt khí nén.