Code:
[i] AudioEnc: [NULL @ 006358c0] Unable to find a suitable output format for 'M:\temp\test.mp4.audio'
With "-codec copy" ffmpeg knows it should keep mp3 codec from input but it is unable to determine a suitable container because you haven't explicitly specified "-f mp3" and it can't guess from the ".audio" file name. (Maybe it makes sense to choose a container that supports all your audio formats, e.g. "-f matroska". You're remuxing anyways.)