Audacity is the number one freebie for editing audio files:
http://www.audacityteam.org/
You need to note that (due to licensing restrictions) the 'out of the box' download can't export files to MP3 format. It's easy to solve that problem though. All you need to do is to download this file as well, and then double-click to run it:
http://lame.buanzo.org/Lame_v3.99.3_for_Windows.exe
(Ignore any 'publisher could not be verified warning' you might see. Don't change any of the options - simply accept the defaults).
You open an audio file in Audacity just as you would open a document in Word. (Go to File > Open. Navigate to the location of your file and double-click on it).
To delete part of a track (such as a pause) in Audacity, simply drag your cursor over the relevant section (to highlight it) and hit the Delete or Backspace key. (See the video below).
When you've got the result you want, DON'T look for a 'Save' option. Instead, go to File > EXPORT in order to find the MP3 option.