Without external binary wrapper is difficult. If you wrap everything in the package is still possible, as well as to define the directory from where you run the binary, here to write - you can already be a problem.
With Windows just all, but for example in Linux application can not even sneeze without authorization. Accordingly, to take and record anywhere will not work.
You can dig in the following areas:
1. Sandbox. In package.json you can specify the path where node-webkit will keep the localStorage, and temporary files, for example:
2. process.env - allows you to get a bunch of data about the environment of the application, including to find information about the tracks. This can be attributed to process.execPath
Likely to have, firstly, to determine the OS, and second, to check access rights to the directory.