Storyline 360 - mp4 file within the project -published web project will freeze when attempting to play the mp4.

Sounds awful Claudia, and I sympathise. Ive been in that situation before with important commercial projects and time critical deadlines.

Thinking back on my years experiencing various video related issues, one idea springs to mind for what might be causing your problem; and re-encoding doesnt fix it just like you said...

a corrupt frame within the video/audio layers can cause the symptoms you describe.

Is the problem only with the one MP4, and is it a local file, with standard playback? Does it have any additional/meta data associated with different channels etc?

Sorry I cant be of more help.