ffmpeg -i audio.m4a -c copy output.mp4
It's literally as fast as copying the file because it's just copying the audio track into a mp4 container.
If you need to upload video, just re-encode it in h264/aac, since that's likely to be the most widely supported at this point. Here's what I use.
ffmpeg -hwaccel cuda -hwaccel_output_format cuda -extra_hw_frames 32 -i video.mkv -c:v h264_nvenc -rc:v vbr -cq:v 30 -b:v 0 -profile:v high -threads 2 -max_muxing_queue_size 9999 -rc-lookahead 20 -c:a aac -b:a 128k -movflags +faststart output.mp4
You might see some error messages scroll past, but I never bothered to figure out what they were, because the output video should play properly. If it doesn't, you probably don't have a supported GPU... in that case use the software encoder instead:
ffmpeg -i video.mkv -c:v libx264 -crf 30 -pix_fmt yuv420p -c:a aac -b:a 128k -movflags +faststart output.mp4