MP3 files are one of the file formats that require the use of FFMPEG.
For that you'll need to download and point Opentoonz to the ffmpeg directory.
Aside: The fork of Opentoonz called Tahoma2D comes with its own build of FFMPEG and so does not need the separate download.
Licensing concerns are the primary reason why Opentoonz does not distribute FFMPEG along with the program.
There are several videos on youtube that walk users through the process of downloading and connecting to FFMEG.
A recent video by DarrenT has up to date information on the process: