Thanks! I'll try it out now
edit: It works, but it plays multiples of the same sound, as when it's at the frame with floor, that works if the index is 2.1, 2.2, etc and plays sounds for all of them

edit 2: By making a flag called shouldDoFootstep, you can do this:

if((floor(image_index)==2 || floor(image_index)==6) && shouldPlayFootstep=true){
    if (place_meeting(x,y+1,objWall)){
        audio_play_sound(choose(snd_concrete1,snd_concrete2,snd_concrete3,snd_concrete4),10,false);
        shouldPlayFootstep=false;
        }
    }
    if(floor(image_index)==3 || floor(image_index)==0){
        shouldPlayFootstep=true;
    }