You *should* be able to do this with sonia, except the library has a bug that breaks the readChannel() function that you would need.  (See my post entitled "Sonia bug: readChannel() not working?")

So I'd reccomend switching to the ESS libary.  The syntax is almost exactly the same.  Anyway, to mix down 3 samples, called s1,s2,s3, try weighted addition into a destination sample.

Code:

 float vol1=1.;  //sample 1 is full volume
 float vol2=.5;  //sample 2 is half volume
 float vol3=.25; //sample 3 is quarter volume
 mixDown=new AudioChannel(441000);  //destination


for(int i=0;i<441000;i++){ mixDown.samples[i]=s1.samples[i]*vol1+s2.samples[i]*vol2+s3.samples[i]*vol3; 
}

 

Of course, in this example the source samples have to be at least as long as the destination sample, so you'd have to perform some checking to make sure you're not going outside array boundaries.