I have made bread using both sponge and preferments. The sponge bread does not taste as sour as preferment bread.  Does preferment add sourness to taste? What other flavors can we expect? What can I do to obtain other flavors excluding sourness?    Preferment (this is what I do): I raise a part of the total dough over night (12 -16 hours). The dough contains only yeast (nothing more). There is no special process. Just letting it sit in a pot overnight or during day for 12 - 16 hours.  I thought sponge is kept for a smaller amount of time as compared with pre-ferment. Other than that I don't know any difference. 
I would add a slight clarification to hoc_age's great answer, since the question mentions using "only yeast" and does not mention sourdough explicitly.  Assuming the question is referring to commercial yeast, a long pre-ferment can still result in some sourness in the flavor.  However, unlike sourdough, the types of bacteria and acids produced may not be as consistent.  Sourdough cultures are selective environments that only grow certain types of bacteria, like certain Lactobacillus strains, which can survive in an acidic environment.  Flour and water mixed with commercial yeast is generally not a particularly acidic environment, so all sorts of things will grow -- and your dough will resemble something like the early stages of a brand-new sourdough starter, with a variety of naturally occurring yeasts and bacteria, many of which live naturally on flour or come from air or water.  Eventually the waste products from the yeast and bacteria will create acids.  In comments, the question of acetic vs. lactic acid came up.  The quantity of acid produced by yeast is not appreciable.  However, bacterial fermentation will produce both acids (and a few other minor ones in very small quantities).  In general, the ratio of these two acids depends on a number of conditions as well as the ingredients.  Fermenting at higher temperatures and with a wetter pre-ferment (e.g., a sponge) will increase the lactic acid production compared to acetic acid, while dry-dough fermentation at lower temperatures will often have a stronger acetic acid taste.  Note that acetic acid is more volatile and thus gives a stronger taste and smell, so not very much of it is required; it also has greater mold-inhibiting properties than lactic acid.  (This is probably the original association of particularly "sour" dough with prospectors in cold temperatures; in those conditions, the bread produced could taste much more sour.)  So it's not only time but temperature and hydration (i.e., "wetness") of the pre-ferment which will effect how much sourness and the type of sour flavors produced.  (In sourdough, the initial amount of starter used in the pre-ferment will also be significant.)  The question asks how to avoid excessive sourness.  Aside from the advice mentioned above -- especially pre-fermenting at warmer temperatures and with higher hydration -- one can also just use a soaker instead (i.e., water and flour, and perhaps other ingredients -- particularly grains -- without yeast).  If the goal is to maximize other flavors beyond sourness, a soaker will allow enzymes to break down the grains and release flavors, but acidity will be less without the yeast.  Another possibility is to use a "mash," which is effectively a soaker heated up somewhat to speed up and maximize enzyme activity.  Sometimes various dough enhancers (e.g., malt powder) can also be used instead or in addition to speed up flavor development. 