Silence removal from the audio Suppose i've a mp3 file. 

I performed FFT of that mp3 file. Then is it good to state the following:
1. Silence occurs by that particular DFT point whose magnitude is lesser 
than some threshold. 2. Inverse FFT gives me the original sound file.
1) No, if you have a particularly low level in one of the FFT points it will indicate that you have low energy in that particular frequency, but you can have several other frequencies at the same time.

2) In theory yes, but in practice the numerical precision will make the resulting signal slightly different than the original one.

Note: before applying the FFT you'll need to decode the mp3 file in order to get the audio data.