Console.log does not mach to array

<html>
    <head>
        <title>lkl</title>
    </head>
    <body>

        <div class="wrapper clearfix">
            <div class="player-panel">
                    
                <button class="btn-answer"><i class="ion-ios-download-outline"></i>Answer</button>
                
                <div onclick="playSounds()">
                    <button class="btn-play"><i class="far fa-play-circle"></i>Instrument Sound</button>
                </div>
                
                <input type="text" placeholder="Instrument name" class="instrument">
                <div id="element"></div>
           </div>
        </div>

    <script>         
    let index;
    let sounds = ["DB.WAV","CLA.WAV","TBN.WAV","REC.WAV","TRI.WAV"];
    oldSounds = [];

    let playSounds = function () {
        index = Math.floor(Math.random() * sounds.length);
        
        console.log(sounds[index]);

        if(index == sounds.indexOf('DB.WAV')) {
            console.log(`Double Bass`);
        } else if(index == sounds.indexOf('CLA.WAV')) {
            console.log(`Clarinet`);
        }else if(index == sounds.indexOf('TBN.WAV')) {
            console.log(`Trombone`);
        }else if(index == sounds.indexOf('REC.WAV')) {
            console.log(`Recorder`);
        }else if(index == sounds.indexOf('TRI.WAV')) {
            console.log(`Triangle`);
        }


        thisSound = sounds[index];

        oldSounds.push(thisSound);
        sounds.splice(index, 1);

        //checking if the code of avobe is working
        console.log(oldSounds);
        console.log(sounds);

        if (sounds.length < 1) {
            sounds = oldSounds.splice(0, oldSounds.length);
        }
    }


    
      </script>
    </body>
</html>