SonicBirth is described as 'visual design software for building Audio Unit plugins. It can be used both as a host within other DAW apps and as a tool to build and export plugins'. There are four alternatives to SonicBirth for Windows, Mac and Linux. The best alternative is Max/MSP. It's not free, so if you're looking for a free alternative, you could try HISE. Other great apps like SonicBirth are SynthEdit (Paid) and FlowStone (Paid).
HISE is a cross-platform open source audio application for building virtual instruments. It emphasizes on sampling, but includes some basic synthesis features for making hybrid instruments as well as audio effects.