Aside from the cell phone-caused speaker interference, there are additional sources of noise/interference in an audio system, including mic feedback. Here are some more resources that describe ways to reduce feedback:
http://toonz.ca/bose/wiki/index.php?title=Microphone_Feedback
http://www.google.com/search?q=mic+feedback
http://en.wikipedia.org/wiki/Audio_feedback#Directivity
http://www.mediacollege.com/audio/howto/feedback.html
http://www.audiocourses.com/article113.html
http://proavmagazine.com/acoustics/designing-a-mix-minus-distributed-sound-system.aspx
http://en.wikipedia.org/wiki/Microphone#Microphone_polar_patterns
http://www.ccisolutions.com/worshiptools/learn-about-microphone-feedback

http://www.shure.com/idc/groups/public/documents/webcontent/us_pro_mics_for_music_sound_ea.pdf
http://www.shurenotes.com/issue23/article.html
http://www.shurenotes.com/issue27/article.html

http://www.dt4u.com/dtsystems/feedback.php
http://boards.straightdope.com/sdmb/archive/index.php/t-34057.html
http://www.behringer.com/EN/Products/FBQ2496.aspx
http://www.mp3backingtrax.com/article56.htm