Audacity is an open source audio editor, very powerful and free. I use it to record my short audio program.
No matter what microphone you use, no matter how quiet your room is, your audio always includes some background noise.
Like this audio above, the selection should be blank, but you can see some wave, because my microphone, and my fan etc. Audience will hear it loud and clear, and nobody will like it.
But we can use Noise Removal Effect built in audience to fix it.
First step, select some background noise like the picture above. Then click menu Effect -> Noise Removal, this dialog box will appear.
Click the Get Noise Profile button, the dialog box will disappear.
Then select the whole audio or what segment you need, and click menu Effect -> Noise Removal again, then click Ok button, and then everything is done.
Now you can see the same section as the first picture, but these is a very clear line now.