ffmpeg/avconv is a command-line tool that converts audio or video formats. It can also grab and encode in real-time from various sources such as a TV card etc.
To extract a part of audio/video file into a new file, you can do the following:
Assuming you have ffmpeg or avconv installed on your system(windows,*nix):
If you have ffmpeg instead of avconv, below commands can be used as is replacing avconv with ffmpeg

Audio:

avconv -i SourceAudio.mp3 -ss HH:MM:SS -t numberOfSeconds -acodec copy DestinationAudio.mp3

Video:

avconv -i SourceAudio.mp4 -ss HH:MM:SS -t numberOfSeconds -vcodec copy -acodec copy DestinationAudio.mp4

In the above line replace the parameters according to your needs:
Here SourceAudio.mp3/mp4 is the source audio from which you want to extract the audio
HH:MM:SS is the start time from which you want to start extracting.
numberOfSeconds is the length of audio you need to extract from start point.
DestinationAudio.mp3/mp4 is the file into which you want to write the extracted audio to.

“You might not know it, but chances are you use ffmpeg in some or the other way everyday”
-Rushi