You need to go to the menu Display->Views and search for the file show of the view product. Then, edit it.
There, near the middle, you will find the code
Code:
<span id="hikashop_product_id_main" class="hikashop_product_id_main">
<input type="hidden" name="product_id" value="<?php echo $this->element->product_id; ?>" />
</span>
Below that code, you can add the code available there
pastebin.com/0Vp9vULK
in order to display the volume of the product
