Audacity is a free tool that is easy to download on your computer.

Audacity Wiki

LAME Encoder
LAME is a high quality MPEG Audio Layer III (MP3) encoder licensed under the LGPL. You will need to also download this tool in order to turn your Audacity files into MP3 files The first time you perform this action, you will need to "connect" or "direct" Audacity to find LAME, so you may want to be sure to save them in the same folder to make it easier. You won't need to perform this action again.