How to convert stereo wav to two mono wav files with libsox/go-sox

Question

I have go-sox successfully splitting a stereo wav file into two files. But there are two issues:

1. Even though the files have the same sampling rate the resulting files are sped up tremendously
2. The resulting two files are still stereo and not mono

The code I am using is here:

https://gist.github.com/jsgoecke/ca8fe5b4e0901ed04ee0

Any ideas on how to get this working with the files playing at original speed and mono? Seems 'remix 1' and 'remix 2' should do just that.

I used brew to install SoX on OSX (v14.4.1_1).