Hi Iain, I don't know your application, but this looks wrong. If you are trying to choose between various microphones, and only output the microphone with the highest priority that has audio on it, (i.e. an automixer), then you're going to have to do a bit more work than this. Here are the immediate problems I see (Chris Cannam already indicated the first 2) 1) 0.0f indicates silence, not -1.0f 2) A microphone input will have some noise on it, you'll need an adjustable threshold value which separates the "on" state from the "off" 3) You can't do this on a sample-by-sample basis, you need some time averaging: 3a) you need to implement a rectified, running average value of all 4 inputs 3b) you'll need to mix the values of all four buffers into the output, with a smoothly changing level control on each input controlled by the logic 4) Make sure the "decay" of the envelope is kinda long ...this will keep a speaker from getting cut off if someody else coughs or whatever There are other subtleties that apply to an automixer, but this is enough to get started. -Ben Loftis