Справку читать, там всё написано.
How do I download and install the LAME MP3 encoder?
Because of software patents, we cannot distribute MP3 encoding software ourselves. Follow these instructions to use the free LAME encoder to export MP3 files with Audacity.
Windows
Go to the [ссылка заблокирована по решению администрации проекта]
Left-click this link, do not right-click.
In "For Audacity on Windows", left-click the link Lame v3.99.3 for Windows.exe and save the file anywhere on your computer.
Double-click "Lame v3.99.3 for Windows.exe" to launch it (you can safely ignore any warnings that the "publisher could not be verified").
Follow the Setup instructions to install LAME for Audacity, making sure not to change the offered installation location of "C:\Program Files\Lame for Audacity" (or "C:\Program Files (x86)\Lame for Audacity" on a 64-bit version of Windows).
You should now be able to export MP3s without any further configuration, choosing File > Export... then selecting "MP3 Files" in the File Export Dialog.
Т. е. сходить по адресу http:// lame1 .buanzo. com.ar/ (пробелы убрать) , скачать установочный файл и установить согласно вышеприведённой инструкции.