Here variable "i" in soundsArray[i], not taking any values, line 4
   var soundsArray = ["crash", "kick-bass", "snare", "tom-1", "tom-2", "tom-3", "tom-4"];
    for(var i = 0; i<document.querySelectorAll(".drum").length; i++){
        document.querySelectorAll(".drum")[i].addEventListener("click", ()=>{
            var audio = new Audio(`./sounds/${soundsArray[i]}.mp3`);
            audio.play();
        });
    }  
   //document.querySelectorAll(".drum").length = 7
 
     
     
    