This effective way of importing does not work with the current version of Musicmaker
Err, yes it does! I've just tested it on my installation of MM 2021 Premium. Initially the file must be converted to a .wav by MM (this is the same for ANY audio program which is handling .mp3 files) so there is an apparent "delay" before the file is "seen" on the timeline.
So, if oyu are not seeing this happen please give us a lot more details about the exact version and variant of MM which you have, your computer, its OS and hardware. Is this problem something you see with ALL mp3 files or only some? Can you post somewhere (not on the forums) one of these files for other users to download and test on their machines?