the sample-streamer-multi example is doing exactly this. The mixing happens at these lines (note the +=):
float out = 0;
for(int i=0;i<NUM_STREAMS;i++) {
// get samples for each channel from each sampleStream object
out += sampleStream[i]->getSample(channel);
}
// you may need to attenuate the output depending on the amount of streams playing back
audioWrite(context, n, channel, out);
If this is not sufficient for your application, would you mind sharing more details, including your previous experience?
