@atia_kabir , in my experience there's currently not any way to use the Canvas APIs to make Mute Assignment default to "true." You're right that Assignments API muted ("don't show grades/assignment feedback yet") is a property of an assignment. However, in the current design of the UI for Canvas, Mute Assignment must currently be applied per assignment by a teacher or TA who can edit grades; my experience has been that the current API behavior reflects that.
I believe you have found one of these already, but for other non-developers who see this and are seeking a feature enhancement, here's a list of the most closely related currently-open feature ideas:
Also, you may be interested in part of a recorded InstructureCon 2016 session, Gradebook Futures: Kill All the Clicks!, in which the awesome Gradebook product manager talks about a Post Policies feature that -- to me -- might provide a different way to have the effect of a Muted default. Ideally, Post Policies might be the place which manages this per-assignment muted property as a course-level default.