Some questions are best left rhetorical. The documentary film “The Heart is a Drum Machine” directly addresses the mystery at its core. Turns out musicians and actors — George Clinton, Isaac Brock, Elijah Wood and more — are as inarticulate as the rest of us when asked, “What is music?” The film premieres at the Northwest Film Forum tonight, when the filmmakers will be on hand and Olympia singer-songwriter Kimya Dawson (“Juno”) will perform. 8 tonight at the NWFF. See the preview below.