Including media in android_assets or android.resource with phonegap build

I'm using the Ionic framework with Phonegap Build, and am trying to use a custom sound with katzer's local notification plugin. I have notifications appearing in the notification center for iOS and Android, and am now trying to play a custom sound for the notification (documenation). I'm not incredibly knowledgeable about the build process for android, so this may be a naive question. I know about /platforms/android/res, but I'm asking specifically about Phonegap Build, which only takes your /www folder. Before I go down the wrong path, and testing things that ought not work, how should I add media assets to android_assets, or android.resource with phonegap build?

The plugin allows for accessing android_assets, or android.resource, and to the best of my understanding, the assets file is the parent of the project's root /www/ directory. I tried and have not been able to access a .mp3 I stored in /www/audio directory and I'm seeing an error in logcat that the media player can't find the file (assume I'm doing it wrong, or issue with the plugin), but I'm unaware of how I should get those resources to the correct directory on device in the first place. Any help?