By definition wouldn't the phase difference be between only two sources no matter what? For any point you could look at all of the phases of waves at that point and try to check for deviations from their mean, but I don't think that the problem as you've defined it can be generalized to multiple sources....

I think what you'll end up wanting is the average amplitude of the (superposed) sound wave at your point, which you can calculate if you've taken Calculus and think about it. Good luck