Write SQL query to solve given problem: What proportion of the total gross of all movies is from movies with songs?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT CAST(COUNT(CASE WHEN T1.song IS NOT NULL THEN T2.movie_title ELSE NULL END) AS REAL) * 100 / COUNT(T2.movie_title) FROM characters AS T1 INNER JOIN movies_total_gross AS T2 ON T1.movie_title = T2.movie_title