No, the MP3 format is not supported. There are better formats available, like the supported OGG Vorbis and AAC m4a formats. These are the two audio formats that MV supports itself. MP3 files will technically work, as most browsers support the file directly, however you can't add them in through the editor, you'd have to set them to play by manually editing the JSON files that the editor creates, and that will not be a fun experience for you, I guarantee it.
You'd also have to edit one of the default scripts to make sure it loads the proper file format.
Alternatively, a way around this limitation is to name the file .mp3.ogg. This will trick MV into thinking the file is an ogg file, as it only checks the file extension and not the file contents. Keep in mind, however, keep in mind that doing this will mean you won't get any official support for audio fiels not playing, because you are using a unsupported format.