no help on this. please help
MP3 is a compressed audio file, MP4 is a compressed video file. Weird question.
Do you want to import the sound into the video.
mp3 ... sound
mp4 ... video , but the sound on the video could be replace.
To the main question, if you want to convert an MP3 to an MP4, you could use Captivate - import the audio track to the first slide, run it the duration of the timeline...and have the stage be small/nothing, or add an image, or series of images along the timeline... Then publish as MP4.
If you want the MP3 to be an MP4 *before* you bring it in to Captivate, you need to use some other program. If you have Creative Cloud, you could certainly use Premiere. But there are other 'conversion' tools out there, though be wary of spyware.