These will likely be prohibitively expensive given the cost of materials. Considering that the headphones cost $29 and that each magnet (in bulk) costs $0.90, you're looking at headphones that cost $99.20 to produce in materials alone. After labor, fixed costs, and profit, you're looking at a very expensive pair of headphones with bad sound quality.
This is a cool idea but the price of rare-earth metals might be its undoing.