Specifically, you refer to "Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, and teaching them to obey everything I have commanded you."
I must tell you, I see no specific authorization in that passage to use computers and networks for teaching and preaching. This Great Commission tells us to go...it by no means allows us to merely send our words.
So, yes, if I interpret this passage from Matthew as strictly as you interpret the passages from Colossians and Ephesians pertaining to praise music, then using computers to spread the Gospel is an unlawful modification of God's command.
But I don't believe that God ever intended his Commission to be interpreted that way; therefore I must conclude that such an application of the "don't add-don't subtract" principle is wrong. And if it's wrong as it applies to the Great Commission, therefore it might very well be wrong as applied to instrumental music.
In the Great Commission, we are told to go, but we are not told what medium to use in order to do that. It is specific in that we are to go, but it is general in the method. May we use a plane, a car, etc. to get where we are going? Sure, those are the methods to fulfill the command to go. The internet is used to help fulfill the part of the command that says to teach.
In Ephesians and Colossians, we are given a specific command. That command is to sing. There is only one medium that we can use to sing, and that is our voices. We are also told that it should come from our hearts. Mechanical instruments of music are not expedient to the command to sing. They are an additional form of praise that is not authorized in the New Testament.