How to save an audio file I have a video where I deleted the animation part and I only have the sound.

When I try to save the file it saves as .blend, how can I save it as an mp3 file. I have already set the start up file to save as mp3.
Properties-->Render-->Audio and select the format in the windows (mp3 / flac ...) make sure properties-->Render-->post processing--> sequencer" is enable (if sequencer is the audio input)