“Aur chahiye to usi tarah na?’’ Ashok Chaturvedi’s tone is chatty and familiar, the telephone conversation with Someshwar Mishra is in Hindi and this three-minute audiotape is key to the CBI’s evidence against Delhi’s Chief Commissioner of Excise and the chairman of Flex Industries. In fact, the arrests came a day after this tape was handed over to the agency by a ‘‘tested source.’’ It’s the nature of the conversation that led CBI to probe the nexus between the two and zero in right when the money was exchanging hands.