Note that it is only safe to be making graph connections from one thread - so if you're manipulating the Graph at all from the main thread, you could run into race conditions.
What I do is set up all of my Node graph ahead of time, and load the buffers async. Whenever they are ready, I pop them into an available BufferPlayerNode, which you could do with the App's
dispatchAsync() for example.
A minute seems long to me for 1mb of audio files, but then you're on android so I don't really know. You could break it up into loading just the sound effects files first, and the soundtrack second, so that at least you can proceed before the big one is finished. Really FilePlayerNode was designed to do things like play larger files (commonly soundtracks), unfortunate that one isn't working for you.