Why is there an odd smell in mumbai?Is it the humidity?

What you smell are all the things you mentioned and more. In general, the streets are filthy with trash, animal excrement and human waste alike. I'm sure you know, the city is highly populated and hot/humid. When you have that many people and some of those people and the animals are peeing, pooping, spitting, etc. in the streets (and all these odors are magnified by the heat/humidity) what would you expect? I'm surprised you can actually smell anything else.