They are there. See this FAQ

Most people here do as you did and export the WAV and then convert to MP3 using other software. BIAB is limited to the codecs included in Windows which favors WMA over MP3. The MP3 codec included in Windows is poor quality. There are ways to install a higher quality codec and, depending on the OS, many here have had problems getting it to work.

Stick to exporting the WAV...

R