BTW. don't create a thread in the processBlock, you will most likely block the audio thread and create buffer underruns. The golden rule is not to create any object, allocate memory or do anything like it in the processBlock method (unless you know exactly what you do, but avoid it at all cost).
You rather create it in the prepareToPlay method (remember this might be called several times) and only shuffle date in the processBlock into your thread.
And also be warned that you access data from different threads, which needs special precautions. Definitely take the time to watch Timur's talk at the CPP convention: http://www.juce.com/forum/topic/timurs-cppcon-presentation or directly to youtube: https://youtu.be/boPEO2auJj4