Edit: Hi. I don't do a ton with audio in my games and I forget a few years ago they revamped the audio system. Did not know the code you were using were actually functions. @FoxyOfJungle answered your question.
The closest thing I'm aware GMS2 has is
I don't believe there's anything equivalent to "audio_track_length(audio)". One thing you could do, however potentially tedious, is manually add your track lengths in total seconds into an array or ds_list, so you could use
var percent = audio_sound_get_track_position(audio) / sound_lengths[| audio]
I'm not really sure if that would work. I'm just waking up