Psychedelics reliably produce sound-triggered visuals in many users: music breathing through geometry, voices rippling the visual field. The resemblance to chromesthesia is obvious and the differences are instructive.
Induced pairings are inconsistent, session to session and minute to minute, where developmental synesthesia’s defining feature is stability. The experiences also tend to be broad washes rather than the precise, itemized inventories synesthetes report.
The leading interpretation runs through disinhibited feedback: the drugs loosen the gating that normally keeps cross-sensory signals below awareness. Which implies the connections exist in everyone, and synesthetes differ in how much traffic those connections carry by default.