No such file or directory in FFmpeg

You may have double extension of file, your "video.mp4" is in fact "video.mp4.mp4" (look at screenshoot of your Windows folder, README.txt has extension hidden)

I recreated your situation (as best I could) on this computer after I watched your video. I downloaded the same version of ffmpeg (don't know about build, since I am limited to using the 32 bit version), copied the executable from the bin folder into the main one, put an mp4 file called "video.mp4" in there, and used your exact command line parameters, minus the absolute path...

ffmpeg -i video.mp4 -f mp3 -ab 320000 -vn music.mp3

..and it worked perfectly for me. Now, I didn't do it from the same directory structure as you had set up (c:\files\ffmpeg). Do you have ffmpeg installed anywhere else on the computer? Have you considered trying a previous build/version? What about 32 bit vs. 64 bit... or vice versa? Is this the first time you've used this version of ffmpeg? Or... has it worked perfectly up until now? Have you tried renaming the input file (silly, I know)?

figured out the issue.

if you type in dir in the ff prompt window, check what FOLDER it is referencing.

For me it was actually referencing the bin folder, not the folder the ffmpeg batch file is in.

I put the video file in the "bin" folder and it found it with no issues.