This has worked for me with three different Bold 9000 users:
Pushing down the button above the camera flash
First take off the back door of your BlackBerry Bold phone. Then push down the small plastic button (white or light gray color) that’s located above the camera flash. You need to have a very careful look to see it, as it is very small. There's a small opening above the camera's flash and it is roughly 1 inch long and 1 mm tall. The button that you need to push down is located right in the middle of that opening. You may push it down using your finger nail or a little pair of scissors. Once you do it correctly your sounds will be loud and clear.