I have Samsung galaxy s duos 2 GT-S7582. my problem is that, everytime I receive a txt message, I don't hear any alert. ive already checked all the notification settings and all of it were turned on..
Do you hear ringing when you receive a call? If you hear nothing at all you might want to check your headphone jack, it has a little switch inside that is used for the phone to detect whether headphones are plugged in or not. These switches tend to get stuck from time to time causing the phone -wrongfully- to detect headphones are plugged in, and sending the audio to the jack instead of the built-in speaker.
Only try the below in case you hear no audio at all from your phone's speaker!
First I would try by blowing some air (either with your mouth or lightly pressurised air, like from those keyboard-cleaning spray cans). If this doesn't work you can try loosening the switch by poking around in the headphone jack with a toothpick or something, just be carefull not to bend any of the contacts or something.