I managed to solve the issue by using the HandBrake converter.
I gave it the existing
.mp4 and asked it to give me a new
.mp4 with the following specs:
.mp4 works in all 4 browsers without issues :)
Something about the way Miro Video converter converted the files caused
IE and Firefox to see the audio track only but not the video track. This
was rectified by reconverting using HandBrake converter.