Waveforms generated by your synth don’t have to be simple or random. They can come from recordings. Samplers operate on this principle. Instead of a simple sine or square wave, they use a digital recording as their sound source. Of course these more complex waves can still be pitched up or down (played faster or slower) like the waveforms coming from a standard VCO. They can also be modified in other ways that don’t have much effect on a periodic wave like looping and reversing. Since the recordings are digital, it’s easy for a synth to start modifying the data in the recording at the bit level. This gives you the ability to warp and crush the original recording until it becomes a brand new sound that’s not even recognizable.
An example of a digital audio player in the modular world is the ADDAC101 .Wav Player by ADDAC System. This module plays samples from an SD card and gives you both CV and manual control over several parameters such as loop size, file position and sample rate. It’s even got a build-in VCA and envelope follower.