As CreativeSpiral wrote, it is in misc, last parameter. I find it one of the worst design choices of the rev2 that there is no direct button for the layer volume. You can achieve something more or less close to changing a layer's volume directly by changing the amp env amount, but if you have velocity in the layer, you should also adjust that one. Also, in many cases, this will change the character of the sound to some degree.
The layer volume, as controlled in misc, can actually be controlled through an NRPN message, so you could control it with an external midi-controller. I still plan to do so, but still didn't find out what is the easiest way. Can anyone maybe give a short description of his/her setup for doing so? I would be most interested in achieving this with a small midi controller or phone app.