Here's a quote from a reply by Barry Broskowitz to a question I asked about this a while back.
"The "rules" for swapping sound files are:
· You can put a 5 volt PS2 file only in a 5 volt PS2 engine
· You can put a 3 volt PS2 file in a 3 volt PS2 engine, or in a PS3 engine
· You can put a PS3 file only in a PS3 engine
· You cannot put a PS3 file in any PS2 engine (3 volt or 5 volt).
Barry"