Try VLC: www.videolan.org
Use "Media/Advanced Open file ", add your video file, then click on the dropdown button at the bottom to select "Convert". The conversion profile you want to use is "Audio AAC (MP4)" and you can edit it to check "Keep original audio track". Save as a .mp4 file and you're good. (just be aware that VLC has some trouble when the filename contains single quote or other special character)

For lawful purposes only of course