Clearly, the Church does not refer to the human voice as a musical instrument. So let’s stick to the original question of the appropriateness of drums and electric guitars at Mass.
Clearly the Church accepts without question that the voice is a musical instrument. She doesn’t need to emphasize the point. As my previous explanation explained, it’s other instruments that need to be examined for appropriateness and the Church then specifies usage. Specifically, the Church even references Cantors, who use theire voice as a musical instrument in a specific function.
Let’s not kid ourselves, voice is the ultimate instrument. All other instruments attempt to copy it. As an instrument, acousticians examine it’s properties with other instruments, music schools teach it, and indeed even make you indicate your “instrument” as voice.
There is no question from any authority to deny that the voice is an instrument. But you are right, the question here is appropriateness of two particular types of other instruments. The nature of the voice as an instrument in the Church is a given, which is why chant is at the root of Catholicism’s musical heritage. It is not uniquely human either as Scripture evidences “choirs of angels” with whom we join or voices in the liturgy.
Perhaps this explains the reason why Chant is so important to our heritage. And that is because the musical form is most perfectly realized by the human voice in the form of an instrument. Logically, organ then follows because it also is unique in it’s ability to be a quasi orchestra under an organist’s control. It is rightful referred to as the “King of Instruments” and therefore aptly suited to sustain the singing (or voice). In this way the Church indicates that the voice is a superior instrument to even the king of instruments, namely the organ.
In all these matters, it is quite clear from Church teaching that the voice is the most important instrument in use for the Church. Organ follows second, and all other instruments must be able to follow that function if they are able.
Joe B