The only DS9 episode where the crew were speaking gibberish was "Babel" in season 1. The reason for this was that they had contracted a form of sensory aphasia. It is O'Brien who is unwittingly responsible for the spread of the illness. While repairing a replicator, he unknowingly activates a device hidden on the station by the Bajoran Resistance 18 years earlier.
The virus begins infecting the stations population firstly through the replicators before it mutates into an airborne pathogen. The entire population is infected (although different species experience different incubation periods)and they must find a cure from former members of the resistance before they all start dying.