Modified submixes/submixes.py fix errors

[TaegyunKwon]

Hi ethman, While I utilize the submixes utils, I found some bugs and fixed. I think it's all about the variable names, thus straight-forward.

btw, Thanks for sharing such a great & usable dataset

[ethman]

Awesome. I'm a bit swamped right now, but I will look this over and merge next week. Thanks for checking it out!