D&C 68

Speaking to those that would be called to teach the gospel in to all people in all lands, the Lord instructed -

"...this is an ensample unto all those who were ordained unto this priesthood, whose mission is appointed unto them to go forth—
3 And this is the ensample unto them, that they shall speak as they are moved upon by the Holy Ghost."  (Doctrine and Covenants | Section 68:2 - 3)

For -

"...whatsoever they shall speak when moved upon by the Holy Ghost shall be scripture, shall be the will of the Lord, shall be the mind of the Lord, shall be the word of the Lord, shall be the voice of the Lord, and the power of God unto salvation.
5 Behold, this is the promise of the Lord unto you, O ye my servants."  (Doctrine and Covenants | Section 68:4 - 5)

Generally, scripture is considered "a body of writings considered sacred or authoritative".  In the case of these words, the missionary is not writing but speaking (which could easily be put in writing).   If it comes as moved upon by the Holy Ghost, there is no question that the words would be considered sacred and authoritive.  So it would qualify as by definition as scripture.  But such teachings must fall within the parameters and doctrine taught in the "standard works" or the teaching of the Prophet who holds all priesthood keys.  Anything outside that parameter, in my mind, would be false in the the Prophet is the only one that can receive new revelations for the Church.  This process then would be a means to know if one is speaking by the Holy Ghost or not.

As I read verse four, I interpret this different than most of saints.  I see this verse as a commandment to those that are moved upon by the Holy Ghost.  In other words, when the Lord said "whatsoever they shall speak" they "shall" use the scriptures as a resource.  And by doing so by the Holy Ghost, they "shall" teach will, mind, and word of the Lord which would mean that it is His voice for -

"...I, Jesus Christ, your Lord and your God, have spoken it.
34 These words are not of men nor of man, but of me; wherefore, you shall testify they are of me and not of man;
35 For it is my voice which speaketh them unto you; for they are given by my Spirit unto you, and by my power you can read them one to another; and save it were by my power you could not have them;
36 Wherefore, you can testify that you have heard my voice, and know my words."  (Doctrine and Covenants | Section 18:33 - 36)

Combining these two scripture references would suggest to me that we are to speak and read them in our teaching as moved upon by the Holy Ghost.