asterisk users - Jan 2004 - announcement using Dial

IF I want to play sound files, 
1.) what format should it be? (*.au or *.wav) 
2.) where should it reside? 
3.) what syntax should I follow? Is
exten=>_.,102,Dial(SIP/${EXTEN}@ip,1,tHA(sound.au)) correct? I tried this and
it doesn't work.

Thanks,
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