If you need to get the SonataMedia public urls exposed through the api

InjectMediaUrlsListener.php

<?php

namespace Application\Sonata\MediaBundle\Listener;

use Application\Sonata\MediaBundle\Entity\Media;
use Doctrine\ORM\Event\LifecycleEventArgs;
use Sonata\MediaBundle\Provider\MediaProviderInterface;
use Sonata\MediaBundle\Provider\Pool;
use Symfony\Component\DependencyInjection\ContainerInterface;

class InjectMediaUrlsListener
{
    /**
     * @var ContainerInterface
     */
    private $container;

    /**
     * @param ContainerInterface $container
     */
    public function __construct(ContainerInterface $container)
    {
        $this->container = $container;
    }

    /**
     * @param LifecycleEventArgs $args
     */
    public function postLoad(LifecycleEventArgs $args)
    {
        $entity = $args->getEntity();
        if ($entity instanceof Media) {
            $this->injectMediaUrls($entity);
        }
    }

    /**
     * Generate and inject public URLs for the given Media entity.
     *
     * @param Media $media
     */
    private function injectMediaUrls(Media $media)
    {
        $pool = $this->getPool();
        $context = $media->getContext();
        $formats = $pool->getFormatNamesByContext($context);
        if (null === $formats) {
            return;
        }

        $provider = $this->getProvider($media->getProviderName());

        $publicUrls = array();
        foreach ($formats as $format => $options) {
            $format = $provider->getFormatName($media, $format);
            $options['url'] = $provider->generatePublicUrl($media, $format);

            $publicUrls[str_replace($context.'_', '', $format)] = $options;
        }
        
        if ('sonata.media.provider.file' === $media->getProviderName()) {
            $publicUrls['reference'] = $provider->generatePublicUrl($media, 'reference');
        }

        $media->setPublicUrls($publicUrls);
    }

    /**
     * Lazy-load the media pool from the container.
     *
     * @return Pool
     */
    private function getPool()
    {
        return $this->container->get('sonata.media.pool');
    }

    /**
     * Get the provider service by name.
     *
     * @param string $name
     * @return MediaProviderInterface
     */
    private function getProvider($name)
    {
        return $this->container->get($name);
    }
}

Media.php

<?php

namespace Application\Sonata\MediaBundle\Entity;

use AppBundle\Interfaces\EtagProviderInterface;
use AppBundle\Traits\EtagProviderTrait;
use AppBundle\Traits\SerializeModelNameTrait;
use Hateoas\Configuration\Annotation as Hateoas;
use JMS\Serializer\Annotation as Serializer;
use Sonata\MediaBundle\Entity\BaseMedia as BaseMedia;

/**
 * This file has been generated by the Sonata EasyExtends bundle ( https://sonata-project.org/bundles/easy-extends ).
 *
 * References :
 *   working with object : http://www.doctrine-project.org/projects/orm/2.0/docs/reference/working-with-objects/en
 *
 * @Serializer\AccessorOrder("alphabetical")
 *
 * @Hateoas\Relation(
 *      "self",
 *      href = @Hateoas\Route("api_get_media", parameters = {"id" = "expr(object.getId())"}, absolute = true)
 * )
*/
class Media extends BaseMedia implements EtagProviderInterface
{
    /**
     * The unique identifier of the Media.
     *
     * @var int
     */
    protected $id;

    /**
     * @var array
     */
    private $publicUrls = array();

    /**
     * Get id.
     *
     * @return int $id
     */
    public function getId()
    {
        return $this->id;
    }

    /**
     * @param $publicUrls array
     *
     * @return self
     */
    public function setPublicUrls(array $publicUrls)
    {
        $this->publicUrls = $publicUrls;

        return $this;
    }

    /**
     * @return array
     */
    public function getPublicUrls()
    {
        return $this->publicUrls;
    }

    /**
     * @param $format string
     *
     * @return null|string
     */
    public function getPublicUrlByFormat($format)
    {
        if (!isset($this->publicUrls[$format])) {
            return null;
        }
        return $this->publicUrls[$format]['url'];
    }
}