update library items

I'm sorry i dont' understand your question.
if the sound files aren't stored in the library then they're loaded at runtime from a location. So do you mean you want to update the location? If so do you want this done at runtime?
if so set a variable for the filePath then create a textfield or somethign where they can type in the new path and then have the button to submit this. but this will only exist for that session it wont' be stored.

Did you take the .fla file from the client ?
Are there any supporting files to the fla or is it a single file ?
can you hear sound when you export movie from clients file ?

If you can hear the sound when you export your movie,
the soundclip will be there somwhere in the library.
sort the library items by 'type' and look for sounds (with the speaker icon.)

once you find the desired sound file to replace,
you need to right click the file and go to properties > import
and select the new sound.

Since the original fla is not yours,
you cannot 'upate' the sound file because the the fla would not find the file path in your machine.

OK I have already have the answer. I must import using proerties from library item . In this case everything will be ok.

zarehb,
If you overlooked the same solution in my answer and discovered the solution by yourself,
Please assign the points to yourself and close this question.