Does the M4V file type have direct support for Audio?
I find the m4v extension a bit confusing.

For MPEG-2, the .m2v extension is associated with a raw video stream. Those files can have only video.

Now for MPEG-4, Apple uses the .m4v extension as a container format, and in fact if you rename an .m4v file to .mp4 it is likely to play on players that only read .mp4 files, since the two format are essentially equivalent.

I think you would need to check this .m4v file and make sure it is really the container format, and not an elementary video stream similar to what you find in .m2v files for MPEG-2.

If you feel comfortable inspecting files with a hex editor, just open this .m4v file, skip the first four bytes and check if the next four are the letters 'ftyp'. If they are, then you have an MP4 container file that should have no trouble holding audio along with the video. If, on the other side, you've got something else, then you may have an elementary stream, in which case you may want to recommend to your vendor to use a different extension for these files. Using .avc is probably more appropriate for an H.264 elementary stream.