Add mix support

[carlthome]

If an audio effects chain is called with a list of audio files or list of ndarrays, they should be mixed together with SoX.

[dsouza95]

Hello,
I would like to apply some effects to two different audios (the actual audio, and an audio containing background noise only) and mix them together.
It seems that this enhancement could be exactly what I am looking for. @carlthome, was there any progress on this?
I have implemented a naive solution to this myself, but it turned out to be very inefficient... Any help with this would be greatly appreciated!