2009-04-08 00:43:24 [ERR] mod_sndfile.c:194 sndfile_file_open() Error
Opening File
[/usr/local/freeswitch/sounds/en/us/callie/conference/conf-pin.wav]
[System error : No such file or directory.]
2009-04-08 00:43:24 [WARNING] mod_conference.c:4799
conference_function() Cannot ask the user for a pin, ending

This is curious. Do you see this error about the missing file when you use PCMU?
-MC

call2009-04-08 00:43:24 [NOTICE] mod_conference.c:4800
conference_function() Hangup sofia/internal/723328@sip.mydomain.com
[CS_EXECUTE] [DESTINATION_OUT_OF_ORDER]