I see why you are confused now. Forget about the value attribute in HTML, this sets the default value for the field. You need to use the name attribute - the nam,e you give a field in the name attribute will be the name you use to access it in your PHP.
HTML Code:
<input type="text" name="music" />
PHP Code:
if (! in_array($_POST['music'], $array)) {
    $array[] = $_POST['music']; // add the value to the array
}