One of the crucial, often undervalued elements of a film is its soundtrack. A good one sets the tone and the mood, mirroring the goals of the filmmaker. Pivotal to that often is the closing credits song. If done well, it can give you a lump in your throat or leave you feeling happy, perhaps contemplative. Emeryville-based Pixar found an ideal Oakland musician/artist to create a song to match the spirit and style of their delightful, quietly revolutionary new animated short “Twenty Something.” Astu’s song “Hold the Line” lyrically conveys the themes of the Disney+ release, part of the Pixar SparkShorts series.