How can you change the environmental factors to change the proportions of sourdough microbiological cultures?
There are a variety of theories about what the most important factors are to control the sourness of a sourdough bread.  Many times you will find conflicting evidence from different sources.

General Considerations


Some starters are naturally more sour.  Some organisms produce more tart flavors, while others produce buttery notes. Some combinations of yeast and bacteria work together and push the process toward acid production, while others compete for resources (Article 3, page 21-22) yielding less acid.
Fermentation time counts.  Anything you do to increase the amount of time it takes to complete bulk fermentation will likely increase sourness.  Usually this is done by using a small amount of starter.  You can also increase sourness by degassing the dough and letting it rise again.
Flour type matters.  The ash content of the flour contributes to buffering the acidity and extending the flavors of the end product.  Higher ash content flours allow the microorganisms to keep working longer.  In general, ash content is directly related to extraction rate. Also, some starters "like" certain flours better than others, this is especially true for whole wheat, rye, or other specialty grain flours. 
Fermentation temperature is tricky.  I have read many sources that go on and on about temperature and sourdough.  Some claim a lower temperature increases sourness, others claim that it decreases sourness.  I tend to find that proofing in my fridge increases sourness somewhat, but also makes rising take an incredibly long time.  According to some sources (page 22), many of the bacteria in sourdough thrive at temperatures between 85-105˚F, while the yeasts thrive at 75-80˚F. Strict control of temperatures within one of these "optimal" ranges can alter the mix of organic molecules produced by a given culture, but most home bakers do not have the equipment needed to maintain such specific temperature tolerances throughout bulk fermentation.


Articles


Squeeze more sour from your sourdough
How to develop sour flavor in sourdough
Sourdough: a tool for the improved flavour


Tip

No matter whether you want it more sour or less sour, always start with an active culture.  The microorganism milieu of your culture is dynamic.  If you try to start a loaf with a starter that is only semi-awake your results will be unpredictable and unlikely to be replicable.