JithinSK- It's actually a limitation of our phone because of which it can not sample sound frequency beyond 22 Kilo Hertz. An audio track on a typical AudioCD (ACD) is recorded at 44KHz and sound in a DVD is at 48KHz. To give you an idea, a telephonic conversation employs a sampling rate of 8KHz, now compare that to the sound of a Audio CD which is sampled at 44.1KHz.
It's actually the rate at which an audio stream or data is sampled per second. Obviously to deliver the same audio quality, the reproducing hardware should also sample the audio data at the same rate at which it is recorded.
If you need to know more about sampling rate, please refer this:
http://en.wikipedia.org/wiki/Sampling_rate
