Hello, pappu886.
Thank you for the question. Here is an article that you may find helpful in troubleshooting your issue.
iPhone: Can't hear through the receiver or speakers
http://support.apple.com/kb/TS1630
Also, make sure to close all applications in multitasking then power cycle the device by press and holding the Sleep/Wake button for a few seconds until the red "slide to power off" slider appears, and then slide the slider. I have included a screen shot of how to close all applications in multitasking.

Cheers,
Jason H.