Disclaimer: I never bothered with the Apple and Samsung law suits.
I'm not sure they will have much on Yamaha if they are gunning for patent infringement (I've not been reading up on the case).
What they should be going for is looking at designs law in intellectual property.
Also, if the issue of a patent challenge has come up, that's the usual counter by anyone who has been accused of infringement. It's more or less general protocol. If Beats get over the line, then the have a case. If they don't, then Yamaha are off the hook. But to prove infringement is a whole other hurdle altogether. You need to find some definitive proof that they have used Beat's technology, some connection that brings them together, to show that they didn't just build the whole thing themselves.
The whole idea behind intellectual property is to protect the process of the idea, at least in patents. But the idea itself is not protected because they don't want to stop "competition" and "innovation" so to speak.
One thing I will say though, if they go for designs, they will have to show that there are a gazillion different ways of making a headphone. And honestly, there are only so many ways you can make a headphone. Sure, Yamaha's ad campaign was targeted at Beats, but you see so many different companies do it, and this doesn't just apply in the audio industry. It applies globally. Alternatively, Yamaha could take the fall back option of a defence based on satire, to some extent. Not sure it is a defence in the country the suit is being brought up.
My little few pence worth.
Far as I'm aware, the lawsuit doesn't involve any utility patents. It's all about design patents and "trade dress."