What specifically were the articles of the sanctuary mentioned in Numbers 31:6?
The scriptures don't give an explicit answer. Some have suggested that the "articles from the sanctuary" were either the Ark, the Urim and Thummim, or just the silver trumpets.
Gill's Exposition of the Entire Bible gives the most comprehensive explanation from Aben Ezra:
by "the holy instruments", Aben Ezra understands the ark with what appertained to it, which in later times used to be carried out when the Israelites went to war, Joshua 6:4, and Jarchi interprets them of the ark and plate of gold (z) which was upon the forehead of the high priest; but what had Phinehas to do with this, who was but a common priest? though the Targum of Jonathan paraphrases it, "with the Urim and Thummim of holiness, to inquire by them;" and it appears, that sometimes a son of an high priest was intrusted with the ephod, to which the breastplate was fastened, which had the Urim and Thummim on it, and made use thereof to inquire by, as in the times of David, 1 Samuel 23:6, but it is the opinion of some learned men, and they may be in the right, that these instruments are no other than the trumpets, and who suppose the "vau" is not copulative, but explanative, so Ben Gersom, and read the words thus, "with the holy instruments, even", or, "that is, the trumpets" (a), the silver trumpets ordered to be made, Numbers 10:2 one of which was for the journey of the camps, and also to blow an alarm for war, and which was done by the priests; and so the Targum of Jonathan adds here, "to cause the camp of Israel to rest, and to cause it to go;'' that is, to direct it when it should stop, and when it should move.