The FFMPEG Utility

mrViewer comes bundled with a copy of ffmpeg, which is a command-line utility used to create movies among other things. The current version supports OpenEXR non-tiled images but only in zip, rle, pxr24 and non compressed modes. The EXRs should also be RGB, not Ycc. You can run ffmpeg from the bin directory of the mrViewer install.
You can create mp4 movie files from images with the following syntax:

ffmpeg -layer Specular -i image.%04d.exr test.mp4

In Windows:

ffmpeg.exe -layer Specular -i image.%04d.exr video.mp4

If not working with a multi-layered openexr file, the command can be simplified to:

ffmpeg -i image.%04d.exr video.mp4

To encode with high quality and with gamma 2.2, and framerate 24, it involves setting other options, like:

ffmpeg -gamma 2.2 -i images.%04d.exr -q:v 0 -r 24 video.mp4

Here -q:v means video quality and -r is the frame rate.

For more information about ffmpeg refer to https://www.ffmpeg.org/ffmpeg.html