Pavlovian Conditioning And Synesthesia
This has been attempted, with some success, though on grapheme-color synesthesia, rather than sound-color synesthesia. Beyond conditioning, there have also been somewhat successful attempts to induce synesthesia through various psychoactive drugs. In both of these cases, however, the effects are temporary and generally weaker than genuine synesthetic perceptions.

There have been no studies (to my knowledge) about which associations can be induced and which cannot. I would say that sound-color might be a difficult association to generate, simply because no one would like to site and stare at a flashing screen while listening to various sounds, and it also predicates on the patients' ability to distinguish sounds, which can vary wildly from person to person. The grapheme-color experiment had the treatment group read materials with certain letters colored, which was much easier to implement.

This raises the question of whether synesthesia can be classified as a low-level interaction between typically non-interacting parts of the brain, or if it operates on a higher cognitive level (i.e. ideathesia). 

See the article for details about the grapheme-color study: 

"Pseudo-Synesthesia through Reading Books with Colored Letters"