There were a number of systems designed for composers in the 90s (also continuing through to today) designed for the workflow of converting a sound to a spectrogram, doing visual processing on the image, and then re-synthesizing the sound from the altered spectrogram. Many were inspired by Xenakis' UPIC system which was designed around the second half of this workflow: you'd draw the spectrogram with a pen and then synthesize it.

https://en.wikipedia.org/wiki/UPIC

Edit: my favorite of all these systems was Chris Penrose's HyperUPIC which provided a lot of freedom in configuring how the analysis and synthesis steps worked.