javascriptで「音が鳴るインターバルタイマー」

<div style="text-align:center;">

   <div class="container mt-5">
      <div id="PassageArea">0</div>
   </div>

   <div class="container mt-5">
      <input type="number" id="number" min=1 value=5 style="width:80px;">
   </div>

   <div class="container mt-5">
      <button class="btn btn-primary" id="startcount" onclick="startShowing();">カウント開始</button>
      <button class="btn btn-primary" id="endcount" onclick="stopShowing();">カウント停止</button>
   </div>
</div>

</div>

<script>
var msg;
var number;
var count = 0;
var timer_id;

function startShowing() {
   count = 0;
   number = document.getElementById("number").value;
   document.getElementById("startcount").disabled = true;
   countup();
}

function stopShowing() {
   count = 0;
   clearTimeout(timer_id);
   document.getElementById("startcount").disabled = false;
}

function countup() {
   count ++;
   document.getElementById("PassageArea").innerHTML = count;
   console.log(count % number);
   if(count % number == 0){
      sound1();
   }else{
      sound2();
   }
   timer_id = setTimeout(countup,1000);
}

function sound1(){
	var id = 'sound-file1';
	if( typeof( document.getElementById( id ).currentTime ) != 'undefined' ){
		document.getElementById( id ).currentTime = 0;
	}
	document.getElementById( id ).play() ;
}

function sound2(){
	var id = 'sound-file2';
	if( typeof( document.getElementById( id ).currentTime ) != 'undefined' ){
		document.getElementById( id ).currentTime = 0;
	}
	document.getElementById( id ).play() ;
}
</script>

<audio id="sound-file1" preload="auto">
	<source src="beep1.mp3" type="audio/mp3">
</audio>

<audio id="sound-file2" preload="auto">
	<source src="beep2.mp3" type="audio/mp3">
</audio>

<style>
#PassageArea{
   font-size:42px;
   border:1px solid #666;
   padding:20px;
   width:100px;
   margin:0px auto;
}
</style>

<audio id="sound-file1" preload="auto">
	<source src="beep1.mp3" type="audio/mp3">
</audio>

function sound1(){
	var id = 'sound-file1';
	if( typeof( document.getElementById( id ).currentTime ) != 'undefined' ){
		document.getElementById( id ).currentTime = 0;
	}
	document.getElementById( id ).play();
}