A sound bite is "a short and easily remembered line, intended by the speaker to be suitable for media repetition."
Reagan and others had success in obtaining media exposure because they gave the media what it wanted. This success lead to a lot of marketing and speech writing activity. Some events were staged to specifically provide quotable lines for media exposure, often just in time for TV newscasts. The role of PR professionals in crafting 'sound bites' that dovetailed with a political strategy helped drive public commentary on the rise of spin doctors. Sound bites are now common usage. 
- Sound bite, The Phrase Finder, accessed December 2007.