Hi there,
I've also got a JDM model and looked into this. It's an air resonator chamber. Much like the central exhaust resonator, the idea is that it reduces particular sound frequencies (for example, there may be a particular RPM band that produces vibration or noise, which Nissan decided to tune out). I experimented by taking it off and plugging the hole, but honestly it seemed to make zero difference to sound or performance. I noticed that the USDM models don't have these, so whatever Japan was trying to tune out must not have been an issue there.
I hope that's helpful.