You can find a couple of them at Apple's OS X downloads page under docklings. All of them have slightly different features. There is one, DVolume, I think, which comes up whith a message saying that it might not work every time you start up, but it works, and allows you to change both system and alert sound levels. It doesn't give you precise control though and has the annoying message. personally I use Volume Dockling, I think..., which only allows you to control system volume, but it gives you precise (or almost precise...) control over it, and doesn't have the annoying message about compatability. There is also a couple others that I haven't tried.