I bought two of these http://www.amazon.com/eForCity-Headp...YAEY1VWGFACB03
IPhone 3.5 to 3.5 slim headphone adapters for my Nexus 5 and Nexus 7. I use the Spigen Neo Hybrid case on my Nexus 5 which has a small headphone jack opening.
With this adapter I can use my Sennheiser 280 Pro headphones for music in which the jack is too big for the case and my headset and mic for calls work with it as well.
And yes, I shuddered when I first sought a solution in these forums and it was suggested I cut out a bigger opening in my case with a razor blade.
Here is a picture of it on my Nexus 7 (took the picture with my Nexus 5) for those who would rather not click the link.