Join new table with SQL query

Always escape/prepare your data

Don’t leave it open to injections:

global $wpdb;
$all_posts = $wpdb->get_results( $wpdb->prepare(
    "
        SELECT %s.*
            FROM %s 
            LEFT JOIN %s ON(%s = %s)
        WHERE %s="soundcloud"
        AND %s="%s"
    "
    ,$wpdb->posts
    ,$wpdb->posts
    ,"{$wpdb->prefix}soundcloud"
    ,$wpdb->posts.ID
    ,"{$wpdb->prefix}soundcloud.idpost"
    ,$wpdb->posts.post_type
    ,$wpdb->posts.post_name
    ,$name
), OBJECT );

echo '<pre>'.var_export( $all_posts, true ).'</pre>';

Use the prefix

$wpdb; offers the $wpdb->prefix, which would be whatever you set in your wp-config.php as table prefix. Then append your table name: {$wpdb->prefix}{$your_table_name} or "{$wpdb->prefix}soundcloud".