The four 16v 10uF capacitors on the top left are the ones that are related to the sound chip. (http://450.servehttp.com/reference/caps/, the 2nd picture from the bottom.) Replacing the 10uF capacitors have resolved "squaking" noises in both my Performa 450 (LC III) and LC II.
If the sound is low, then you also need to replace the 6v 100uF.
If the sound is low, then you also need to replace the 6v 100uF.