My mobile speaker not working

@dipankarsardar you are really not giving us much to work with here. It would help us to know what you have checked and what you have already tried. Also, let us know if there was anything that preceded this. Things like if it got dropped or has water damage etc. are all-important.

Start by ruling out a stuck headphone sensor. Pocket lint and debris inside the headphone jack can trick the mechanical switch into thinking headphones are plugged in. That would cut power to the loudspeaker. Clear out any debris with a a few puffs of Air-in-a- Can. Next check your audio routing in Settings under Sound and Vibration to ensure media volume is maximized and Bluetooth is off. Also check Gestures and Motions and disable the flip-to-mute feature.

If all that checks out you will need to remove the speaker. Yes, of course, you can check it first so that you know it is the speaker and not some issue on the motherboard, which is always a possibility. Since iFixit does not yet have a specific guide for that use something like this video. Inspect the two spring contacts on the buzzer housing for oxidation, ensuring they press firmly against the contacts on the  sub-board. Set a digital multimeter to resistance and probe the two contact pads on the loudspeaker module. A good voice coil reads between 6 and 8 Ohms. An OL reading would be a blown  coil. In those cases you will need to replace the speaker.

If the speaker tests ok, signal delivery from the mainboard is failing. Reseat both ends of the main FPC flex cable connecting the sub-board to the motherboard,. Check the flex cable for any micro tears, corrosion etc.

Ultimately I suggest you start with a new speaker and then reevaluate.

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person who has the same issues with their device. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!