Audacity freeware: Audacity software is a tool for processing audio, so has the help of this free software you can make several changes to a certain level files,realy it's a good free audio editing software.
As a tool for processing,recording and playback of audio files, you can add audicity silence in an audio file, rather than the possibility that mutes the audio level requested by the user, rather than its ability conversion.
On the other hand the audacity software allows you to make unlimited recordings and save them.