Not for the faint of ear, this festival offers daunting music in a friendly, almost familial setting. In this 40th go-round, it features such stalwarts of nearly pure improvisation as William Parker, Joëlle Léandre, Irène Schweizer, Hamid Drake—and you, as you interact with the “sound art” on display. The million-Euro question: which Peter Brötzmann will show up, the Gatling-gun berserker or the broken, forlorn poet glimpsed in recent collaborations with Heather Leigh? —E.E.