<p>At what length of text and/or length of audio snippet does a piece of commercially distributable software pass the threshold of fair use and violate the included work's copyright?  Does attribution absolve the developer from infringement?  An example would be a quote from a novel used on a start-up screen.</p>

Unfortunately, there is no cut and dried answer.  Determining what is fair use involves a very subjective and fact-dependent
four point test
.  You're never really going to know for sure if a borderline use is permissible or not unless you end up in court and a judge decides.
The four factors are:
the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
the nature of the copyrighted work;
the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
the effect of the use upon the potential market for or value of the copyrighted work.
Each of these has a specific legal meaning based on previous precedents (which may or may not correspond to what most people would think of as the plain language meaning).  If you're doing anything that could get you sued, talk to a lawyer.
Software is even more complicated, since not all code is copyrightable to begin with.