Trying to implement https://github.com/sonata-project/SonataMediaBundle/issues/605#issuecomment-51250006

Media.orm.xml

<?xml version="1.0" encoding="utf-8"?>
<doctrine-mapping
    xmlns="http://doctrine-project.org/schemas/orm/doctrine-mapping"
    xsi="http://www.w3.org/2001/XMLSchema-instance"
    schemaLocation="http://doctrine-project.org/schemas/orm/doctrine-mapping
                    http://doctrine-project.org/schemas/orm/doctrine-mapping.xsd">

    <entity name="Application\Sonata\MediaBundle\Entity\Media"
            table="media__media">

        <id name="id" type="integer" column="id">
            <generator strategy="AUTO"/>
        </id>

        <one-to-many field="sources"
                     target-entity="Application\Sonata\MediaBundle\Entity\Media"
                     mapped-by="parent" />

        <many-to-one field="parent"
                     target-entity="Application\Sonata\MediaBundle\Entity\Media"
                     inversed-by="sources" >

            <join-column name="parent_id" referenced-colum-name="id" on-delete="CASCADE" />

        </many-to-one>

    </entity>

</doctrine-mapping>

Media.php

<?php

namespace Application\Sonata\MediaBundle\Entity;

use Sonata\MediaBundle\Entity\BaseMedia as BaseMedia;
use Sonata\MediaBundle\Model\MediaInterface;
use Doctrine\Common\Collections\ArrayCollection;


class Media extends BaseMedia
{
    /**
     * @var integer $id
     */
    protected $id;

    /**
     * @var MediaInterface
     */
    protected $parent;

    /**
     * @var ArrayCollection
     */
    protected $sources;

    /**
     * @param mixed 
     */
    public function __construct()
    {
        $this->sources = new ArrayCollection();
    }

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

    /**
     * Get parent
     * @return MediaInterface
     */
    public function getParent()
    {
        return $this->parent;
    }
    
    /**
     * Set parent
     * @param MediaInterface $media
     * @return $this
     */
    public function setParent(MediaInterface $media = null)
    {
        $this->parent = $media;
    
        return $this;
    }

    public function removeParent()
    {
        $this->parent = null;

        return $this;
    }

    /**
     * Get sources
     * @return ArrayCollection
     */
    public function getSources()
    {
        return $this->sources;
    }
    
    /**
     * Set sources
     * @param ArrayCollection $sources
     * @return $this
     */
    public function setSources(ArrayCollection $sources)
    {
        foreach ($sources as $source) {
            $this->addSource($source);
        }
    
        return $this;
    }

    public function addSource(MediaInterface $source)
    {
        if (!$this->sources->contains($source)) {
            $this->sources->add($source);
            $source->setParent($this);
        }

        return $this;
    }

    public function removeSource(MediaInterface $source)
    {
        $this->sources->remove($source);
        $source->removeParent();

        return $this;
    }
    
}

MediaAdmin.php

<?php

namespace Application\Sonata\MediaBundle\Admin;

use Symfony\Component\Form\FormEvent;
use Symfony\Component\Form\FormEvents;

use Sonata\AdminBundle\Form\FormMapper;
use Sonata\MediaBundle\Admin\ORM\MediaAdmin as BaseMediaAdmin;
use Sonata\AdminBundle\Datagrid\ListMapper;
use Sonata\MediaBundle\Form\DataTransformer\ProviderDataTransformer;
use Sonata\MediaBundle\Model\MediaInterface;

class MediaAdmin extends BaseMediaAdmin
{
    // [... configure list fields ...]

    /**
     * Override Sonata\MediaBundle\Admin\BaseMediaAdmin
     *
     * @param FormMapper $formMapper
     */
    protected function configureFormFields(FormMapper $formMapper)
    {
        $media = $this->getSubject();

        if (!$media) {
            $media = $this->getNewInstance();
        }

        if ($this->hasParentFieldDescription()) {
            // the admin is embedded
            $this->configureSourcesFormFields($media, $formMapper);
        } else {
            $this->configureMediaFormFields($media, $formMapper);
        }
    }

    private function configureMediaFormFields($media, $formMapper)
    {
        $formMapper
            ->add('name')
            ->add('enabled')
            ->add('description')
            ->add('authorName')
            ->add('copyright')
            ->with('Sources')
                ->add('sources', 'sonata_type_collection', [
                   'required' => true,
                   'label' => 'Sources',
                   'by_reference' => false,
                ], [
                    'edit' => 'inline',
                    'inline' => 'table',
                    // needed for the Add button
                    'link_parameters' => [
                        'context' => $this->request->get('context'),
                        'provider' => $this->request->get('provider')
                    ]
                ])
            ->end()
        ;
        $formMapper->getFormBuilder()->addEventListener(FormEvents::POST_SUBMIT, function(FormEvent $event) {
            if ($this->request->isXmlHttpRequest() || $this->request->get('_xml_http_request')) {
                return;
            }
            $toSource = [
                'name', 'enabled', 'providerName',
                'context'
            ];
            $toMedia = [
                'providerReference', 'providerStatus'
            ];
            foreach ($media->getSources() as $source) {
                foreach ($toSource as $prop) {
                    $prop = ucfirst($prop);
                    $get = 'get' . $prop;
                    $set = 'set' . $prop;
                    $source->$set($media->$get());
                }
                foreach ($toMedia as $prop) {
                    $prop = ucfirst($prop);
                    $get = 'get' . $prop;
                    $set = 'set' . $prop;
                    $media->$set($source->$get());
                }
            }
        });
    }

    private function configureSourcesFormFields($media, $formMapper)
    {
        $parent = $media->getParent();

        if (!$parent || !$parent->getProviderName()) {
            return;
        }

        $transformer = new ProviderDataTransformer($this->pool, $this->getClass());
        $formMapper->getFormBuilder()->addModelTransformer($transformer, true);

        $provider = $this->pool->getProvider($parent->getProviderName());

        if ($media->getId()) {
            $provider->buildEditForm($formMapper);
        } else {
            $provider->buildCreateForm($formMapper);
        }
    }
}

services.yml

services:
    sonata.media.admin.media:
        class: Application\Sonata\MediaBundle\Admin\MediaAdmin
        arguments:
            - ~
            - Application\Sonata\MediaBundle\Entity\Media
            - %sonata.media.admin.media.controller%
            - @sonata.media.pool
        tags:
            -
                name: sonata.admin
                manager_type: orm
                group: sonata_media
                label_catalogue: "%sonata.media.admin.media.translation_domain"
                label: Medias
                translator_strategy: sonata.admin.label.strategy.underscore
        calls:
            - [setModelManager, [@sonata.media.admin.media.manager]]
            - [setTranslationDomain, [%sonata.media.admin.media.translation_domain%]]
            - [setTemplates, [{list: SonataMediaBundle:MediaAdmin:list.html.twig}]]
            # Custom templates
            - [setTemplate, [list_custom, ApplicationSonataMediaBundle:MediaAdmin:list_custom.html.twig]]