Display multiple subtype objects in one page

I have a base class with 3 sub ones: Item <-- Book, MusicCD, Software

They have some common and different properties but in the view I want to get a mixed list of them and show in one table:

Title Price Volume Author Artist Edition Version

So I have done this:

Classes:

abstract class Item {
    protected $id;
    protected $title;
    protected $price;
    function getTitle() { return $this->title; }
    function getPrice() { return $this->price; }
    function getAuthor() { return ''; }
    function getEdition() { return ''; }
    function getVolume() { return ''; }
    function getArtist() { return ''; }
    function getVersion() { return ''; }
}
class Book extends Item {
    private $author;
    private $edition;
    private $volume;
    function getAuthor() { return $this->author; }
    function getEdition() { return $this->edition; }
    function getVolume() { return $this->volume; }
}
class MusicCd extends Item {
    private $artist;
    private $volume;
    function getArtist() { return $this->artist; }
    function getVolume() { return $this->volume; }
}
class Software extends Item {
    private $version;
    function getVersion() { return $this->version; }
}

View:

<?php
// The controller pass items to view
$items = $this->items;
?>
<table>
<tr>
<th>Title</th><th>Price</th><th>Volume</th><th>Author</th><th>Artist</th><th>Edition</th><th>Version</th>
</tr>
<?php
foreach ($items as $i) :
?>
<tr>
<td><?=$i->getTitle()?></td>
<td><?=$i->getPrice()?></td>
<td><?=$i->getVolume()?></td>
<td><?=$i->getAuthor()?></td>
<td><?=$i->getArtist()?></td>
<td><?=$i->getEdition()?></td>
<td><?=$i->getVersion()?></td>
</tr>
<?php
endforeach;
?>
</table>

Here the question: I feel that the classes seem not good enough because if there are more subtypes appears, chance that I have to modify the Item class. Please help me improve that.