Hey Louise, didn't we have Subotnick's class together last year? In any case, I think that most of what you're going to find is in the realm of modular synth emulation, something like Aalto or FlowStone. You could also argue that using something like Max for Live is using visual tools to make sound, depending on how you construct the instrument. There's also something similar from Native Instruments, called Reaktor, that can be loaded from a plugin. I've seen a few things that use webcams for interfacing as well, but those tend to be built on top of Max for Live using Jitter and MSP. Checking out some of the NIME papers might be worthwhile as well, though from what I know about that conference (which is, admittedly, very little) a lot of that tends to be more geared towards hardware interfaces instead of software.