“Deborah and Barak the son of Abinoam sang.” The verb “sang” is a feminine singular verb even though Deborah and Barak are two people. This a one of the many cases where a plural subject takes a singular verb. The verb is likely feminine because Deborah is mentioned first and the women are emphasized in Judges 4 and 5.
“sang.” It is hard to know exactly how they “sang.” The word for “song” and “poem” are the same word in Hebrew. Whether they “sang” or “spoke” the words is not known.