rocknbil - 8:05 pm on Mar 31, 2009 (gmt 0) Dig around the Sound() class in the AS3 manual of the help files.
I think you might have to do a bytesLoaded/bytesTotal check as you won't know the length until it's completely loaded. When bytesLoaded=bytesTotal, then you might be able to query for length, which will give you length in milliseconds. With .flv files, this is inherent in the file (presuming in the file headers) so you can get a video length as soon as it loads, but appears to be different with the Sound() object.
Dig around the Sound() class in the AS3 manual of the help files.