"The Bible only says to sing! Using a banjo is unauthorized." Is a person playing a guitar while making vocalizations somehow not singing? Check this out from God's word:
"Sing to the LORD with thanksgiving; Sing praises to our God on the lyre!" (Psalms 33:2)
God never defined singing as "without an instrument." In fact, He instructed us to sing Psalms, and the above is one.