Per the official readme...the sound buzz is the only thing that revision 1.1 was released for
And yes, as mentioned above similar fixes were released for other titles around that time.
I'd be happy to burn one (or other games) for any of you that dont' have a rom burner. Just send me a PM so I don't hijack the thread.

Congo - Sound Rom S2 Revision 1.1

This revision eliminates a 'buzzing' sound that is occaisionally
emitted from some games. If you experience this problem,
use this file to create a new S2 and replace the existing S2 on
the Audio Visual board in your game.

Revision Info

CGS2V1_1.ROM Version 1.1 Checksum = $9300