Foreign Accent
In our daily life、we are aware of the existence of accent、especially when having a conversation with foreigners from other countries. For example、we can identify a person whether his is from China or Taiwan by the pronunciation of words with retroflex sounds.

The first website below is a phonetic database of different language speakers reading the same article. The other is a clip about one man who can imitate about 100 people’s voice、which shows that every people has different way of speaking and、of course、different voice quality.

The speech accent archive (QuickTime required)
The Man of Another 100 Voices

In some special cases、people starts to speak with another voice and tone、like he or she is possesed by some demon. In fact、it's your brain again. Visit these websites below to get further information.
Foreign Accent Syndrome
Possession? No、brain damage