It is extremely unlikely that some dedicated tmp to mp4 converter exists. Temporary files, in general, are generated for many purposes and even if they would contain (at least partially) audio data, there is no real way how you could convert it.
You can always try to rename your .tmp file to .mp4 and see if that works. Otherwise you would need to analyze your file with programs like the FileAlyzer and determine what it actually is. In most cases it will be just chunks of data unsuitable for any conversion.Open TMP file Open MP4 file
A free utility for file analysis