How does the use of alliteration add to the mood?
By emphasizing certain sounds, the writer helps create the mood that s/he wants. Check this site for examples of various moods helped by alliteration.
http://www.studyzone.org/mtestprep/ela8/a/lalliteration.cfm
Ponderous pachiderms parade across the public plazas in Peru. <G> Yes, depending on the sound of the alliteration, it can be soothing, humorous or add to other feelings the authors wishes to provoke.