How to convert .mp1 audio to .mp4 format.
Converting MPEG-1 Layer 1 Audio Files directly to MPEG-4 Videos (.mp1 to .mp4 conversion) should be possible, even though your options might be limited. It relates to users attempting to encode MP1 audio to MP4 video.
Try using a media editing software or (online) conversion tool, such as Tiny Audio Converter, which should allow users to import MP1 audio and export it in MP4 format.
Audio converter for Mac