To export as MP3 in 1.3.7 on Windows, you MUST update to the latest lame_enc.dll by obtaining the latest LAME .exe installer from http://lame.buanzo.com.ar/ . Instructions are here. Otherwise, you may receive an error that you do not have the correct version of lame-enc.dll, or get crashes after MP3 export completes.
This requirement is noted in the README.txt accompanying 1.3.7, and on:
where it says "Windows users will require the latest version of the LAME encoder".
The technical explanation of this is that we have rebuilt lame_enc.dll to use the LAME API instead of the older Blade API. This fixes many previous issues such as VBR exports being incorrect, and metadata tags not being seen in a variety of different player applications.