Changing the camera’s sound in N900 is quite easy. All system sounds are stored in usr/share/sound. Just follow the following simple steps to do so:

• Root your N900 if you have not already. Just download rootsh from application manager.
• Connect to N900 using any FTP client (like Filezilla or WinSCP) over WiFi.
• Go to usr/share/sound
• Replace camera_snd_title_1 by any new sound file (the format doesn’t matter). It’s the camera’s shutter sound
• Replace camera_snd_title_2. It’s the video recording sound
• Replace camera_snd_title_3. It’s the camera’s focus sound
• Disconnect and restart your phone

That’s it. This will change your phone’s camera sounds to the new sound files. If you want a silent camera, just rename or delete the files instead of adding new ones.