Background-Info: The main problem is the FileSystem method Filesystem::searchByMime() is only searching threw your own Data. Files that are shares with you aren't searched. As my understanding in OwnCloud logic isn't that deep I decided myself to make this "quick and dirty" approach (that is working fine for me).

owncloud_workaround.php

<?php
/**
 * Workaround for OWNCLOUD that puts SHARED MUSIC into your Player 
 * -> https://github.com/owncloud/apps/issues/855
 *
 * @author: jan1337z <[email protected]>
 *  
 * -> modifications are made in BASE_FOLDER/apps/media/lib/scanner.php
 * 
 * To Do (in scanner.php)
 * 1) Overwrite/Change the getMusic() function with mine below
 * 2) Add those 2 helper functions: getSharedMusic() & getSharedMusicREK()
 * 3) change it for your needs
 *
 * 
 */

        ## IMPORTANT: DO IT AT YOUR OWN RISK - Marked stuff is changed / added ##


        /**
         * get a list of all music files of the user
	 *
	 * @return array
	 */
	public function getMusic() {
		$music = \OC\Files\Filesystem::searchByMime('audio');
		$ogg = \OC\Files\Filesystem::searchByMime('application/ogg');
		$shared = $this->getSharedMusic("//Shared");
		$music = array_merge($music, $ogg,$shared);
		foreach ($music as &$file) {

			$file = $file['path'];
		}
		return $music;
	}
	
	/**
	 * get a list of all shared music files 
	 * @param path to Shared Folder $sharedFolder
	 * @return array
	 */
	public function getSharedMusic($sharedFolderPath) {
    	    $sharedFolder = \OC\Files\Filesystem::getDirectoryContent($sharedFolderPath);
    	    $files = array();
    	    $this->getSharedMusicREK($sharedFolder,$sharedFolderPath,$files);
    	    return $files;
	}
	
	/**
	 *  helper function for getSharedMusic
	 *
	 * @return void
	 */
        private function getSharedMusicREK($array,$path,&$files) {
            foreach($array as $key => $value) {
                if($value["mimepart"] == 'audio') {
                    $value["path"] = $path."/".$value["name"];
                	$files[] = $value;
                	continue;
                } elseif($value["type"] == 'dir') {
                    $this->getSharedMusicREK(\OC\Files\Filesystem::getDirectoryContent($path."/".$value["name"]),$path."/".$value["name"],$files);
              	    continue;
        	}
    	    }
        }
?>

Can confirm that this hack works. Thx