Two Alternative Label->getTrackIDs() methods...

Label::add_method('getTrackIDs', function() {
    $db = static::getDB();
    return array_reduce(
        array_map(function($id) use($db) {
            $query = $db->prepare('SELECT id FROM tracks WHERE releaseID=:id;');
            $query->bindValue(':id', $id, PDO::PARAM_INT);
            $query->execute();
            return array_map(function($id) {
                return intval($id);
            }, $query->fetchAll(PDO::FETCH_COLUMN, 0));
        }, $this->getReleaseIDs()),
        function($a, $b) {
            return array_merge($a, $b);
        },
        []
    );
});

Label::add_method('getTrackIDs', function() {
    $rids = $this->getReleaseIDs();
    $condition = implode(
        ' OR ',
        array_map(function($index, $id) {
            return "releaseID=:id$index";
        }, array_keys($rids), $rids)
    );
    if(!$condition)
        return [];
    $db = static::getDB();
    $query = $db->prepare("SELECT id FROM tracks WHERE $condition;");
    foreach($rids as $index => $id) {
        $query->bindValue(":id$index", $id, PDO::PARAM_INT);
    }
    $query->execute();
    return array_map(function($id) {
        return intval($id);
    }, $query->fetchAll(PDO::FETCH_COLUMN, 0));
});