I'm running Windows 7 Ultimate x64, and I have a USB mixer that doesn't yet have 64-bit drivers. Is there some way to make it work? Would running a 32 bit virtual machine work?
Yes, use any of the virtual machine programs that support USB pass through, and you should be able to use it within the virtual machine using the 32 bit driver.
I do this to use an old scanner within a 32 bit Windows 2000 virtual machine.
Windows XP Mode (which you will be able to use with Ultimate edition) allows USB devices to be passed into the Virtual Machine.
Thus the host Win7 system doesn't need drivers.
(I believe that you could also host another Windows 7 instance -- this time 32bit -- but have not tried this.)