If Arc was an option in speedgrader for teachers to give feedback, it would mean that the teacher could annotate the student's submission and record their voice at the same time.
At the moment, to do this in canvas, they would need to open arc separately, move to speed grader and start to annotate the work.
Doing this work would also allow students to submit arcs as media types.
in the screenshot below, I imagine you would inject either a new tab, or a new button that opens Arc.
Here is a mockup of what this would allow the teacher to do: