Yes, the slider onChange function is for the volume.
However, what have you got in your code that plays the MP3 when the user clicks on the play arrow icon.
If you have not used the Wix Audio Player, then you should have an onClick function so that when the user clicks on the arrow it starts to play the file.
$w("#playbutton").onClick((event) => {
$w("#myAudioPlayer").play();
}
//or//
export function playButton_click(event) {
$w("#myAudioPlayer").play();
}