My best guess is that Chromium's mediocre memory management system (Garbage Collector) isn't doing the job when it's supposed to.
NW.js allows us to expose the garbage collector and trigger it on demand to instantly unload things from memory.
You can find steps to expose GC in my NW.js Roundup, feel free to test out How To: Force GC To Instantly Unload Audio From Memory and share your results in here.
(It's not just unloading audio by the way, it's unloading everything in the "waiting line" which is mostly just audio.)