I think it will be helpful if you put your dialplan entry and the full log on pastebin.freeswitch.org helps others helping you 

-- 
Seven Du
Sent with Sparrow

On Tuesday, August 9, 2011 at 7:12 AM, ran zhang wrote:

i'm trying to play a sound when entering the conference, FS gives an
error that 'switch_core_file.c122 Invalid file format [wav] for
[/sounds/welcomef.wav]

I put '/sounds' as the sound prefix and welcomef.wav as the wave file
for entering conference.

please help!!!
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Attachments:
- welcomef.wav