Some good info I found at MouseSavers...
Source: Tips
Cheerleeding/Twirl/Dance Competitions
Various dates each year, especially January-March. These usually don't have a lot of impact on the in-park crowds. The issue is that the Value resorts (All-Stars and Pop Century) play host to hordes of girls who can be extremely loud, so you may want to avoid those resorts during competitions. Some events take place at Indiana Jones Stunt Spectacular in Disney's Hollywood Studios, so that attraction might be closed. Some of the competitions and events include College Cheerleading & Dance Team National Championships (January 18-20, 2013), National Dance Team Championship (February 2-3, 2013), National High School Cheerleading Championship (February 9-10, 2013), Twirlmania (February 15-18, 2013), Contest of Champions Nationals (March 1-3, 2013), International All Star Cheerleading Championship (March 9-10, 2013), All Star Cheerleading and Dance Worlds (April 27-29, 2013).
South American Tour Groups
In July each year, large groups of teenagers from Brazil and Argentina descend on Walt Disney World for their winter break. Some of the groups do a lot of chanting, singing, clapping and shouting. The sheer size of the groups (up to 100 kids in a single group) can make navigating around them difficult. There have been complaints of tour group leaders taking over the FASTPASS machines first thing in the morning with dozens of tickets in hand, and thus grabbing all of the early return times. The South American tour groups also visit Disney World in January, which is summer vacation in Brazil, but those tend to include more family-oriented groups that do not cause as many issues.