I listen to recorded utterances from people talking to speech IVRs. One thing that people often do when when the IVR doesn't understand them is to change their speech pattern: they'll talk louder or more slowly. If the caller is soft spoken and there's noise in the background, then talking louder will help. Under most circumstances, though, shouting doesn't help, and talking more slowly almost never helps.
If you've ever traveled abroad (or live outside the US) then you may have witnessed US tourists trying to make themselves understood to non-English speaking locals. Often, the tourists will shout and talk more slowly, adding helpful gestures in an amusing pantomime, in order to make themselves understood. In fact, adding gestures can help, and speaking more slowly can help if the local speaks some English, and if it isn't done to the point of changing one's pronunciation.
Having witnessed these two scenarios many times here's my theory of the day: callers to speech IVRs believe the systems to be hearing impaired foreigners who are best communicated with by shouting and speaking slowly to. If you were to watch these people on the phone I'm confident that you'd see the pantomime routine as well.