That could be the case here. Vic may be singing in Federation Standard (aka English) and Kira /Odo have learned it after hanging out with the Federation for the last six to seven years. Considering their backgrounds as people who lived in multi-lingual societies (from the Occupation), it wouldn’t be the strangest thing for them to pick up a new language with some off screen effort over the years.
That could be the case here. Vic may be singing in Federation Standard (aka English) and Kira /Odo have learned it after hanging out with the Federation for the last six to seven years. Considering their backgrounds as people who lived in multi-lingual societies (from the Occupation), it wouldn’t be the strangest thing for them to pick up a new language with some off screen effort over the years.