Most likely the sound files were linked and then not included in the files sent through email. You can learn the particulars here: http://www.pptfaq.com/FAQ00155_Sounds-Movies_don-t_play-_images_disappear_or_links_break_when_I_move_or_email_a_presentation.htm

Having said that, PowerPoint 2010 now embeds sound and movie files in the presentation so this isn't the issue it once was.

Glenna Shaw
MS PowerPoint MVP