How to convert .m2a files to .mp4 format.
Converting MPEG-1 Layer 2 Audio Files directly to MPEG-4 Videos (.m2a to .mp4 conversion) should be possible, even though your options might be limited.
Try using a media editing software or (online) conversion tool, such as Adobe Premiere Pro, which should allow users to import M2A files and export them in MP4 format.
Multimedia encoding application from Adobe
A timeline-based video editing software from Adobe