King James is a translation of the New Testament that tries to stay as close as possible to the Greek original, therefore, you can see some ways in which grammar can be interpreted: "be filled with the Spirit; 19 Speaking to yourselves in psalms and hymns and spiritual songs; Singing and making melody in your heart to the Lord; 20 Giving thanks always for all things unto God and the Father in the name of our Lord Jesus Christ; 21 Submitting yourselves one to another in the fear of God"
All of these participles can be read to mean "by speaking", "by singing", "by giving thanks", "by submitting", etc. This is a possibility, but one should pay respect to the way the author uses the participles. Just one verse above our text, Paul writes "be ye not unwise, but understanding what the will of the Lord is. ", which means "be ye not unwise, but understand what the will of the Lord is." The translation seems strange, because King James, following closely the original used a participle. If Paul wanted to say "be not unwise, by understanding", he could not have the but (ἀλλὰ) that starts the secondary sentence. The same use of the participle continues in our passage, with each participle not meaning a cause, but just starting a new sentence, explaining the goods of the Spirit. This way in each sentence we presuppose the imperative of the verb "to be" (ἐστὲ λαλοῦντες, etc: be speaking, be singing, be giving, etc).
Therefore our passage should be read like this: "Be filled with the Spirit; 19 Speak to yourselves in psalms and hymns and spiritual songs; Sing and make melody in your heart to the Lord; 20 Give thanks always for all things unto God and the Father in the name of our Lord Jesus Christ; 21 Submit yourselves one to another in the fear of God"