There was the Old Testament present at all times in the 1c., including at the Pentecost. The New Testament -- the scripture that gives the Good News -- has not been written.
The descent of the Holy SPirit (vv 2-4) preceded Peter's sermon. But the descent happened to an existing assemply. That descent made the assembly Church.
Something else happened at the Pentecost and before the sermon: the Apostles spoke in tongues. Thus, they were not citing scripture: they were speaking from their hearts to the people, to the point of appearing delirious. Well, that was the Church speaking,-- speaking before she wrote.
Your statement was that there was NO SCRIPTURE at Pentacost. The Old Testament is Scripture. When they spoke in the languages of the people present, they weren't just sharing from their hearts - rather, they were prophesying as the Spirit gave them utterance - as Peter indicates at the beginning of his sermon - tying the event directly with the prophecy of Scripture. It wasn't 'the church' acting apart from Scripture, but the church uttering Scriptural truth. And Peter's own words were no doubt heard as the voices of the others were heard - in the language of the hearer. And those very words were laden with Scripture. The words of Peter's sermon gained their authority from the direct quotation of Scripture. Peter didn't reference any supposed "Super-apostolic" authority; rather, he pointed to Joel, and David, and the miracles of Christ Himself as the authority for the words he was speaking. In other words, Peter was a tremendous example of Sola Scriptura and Solos Christos in his sermon at Pentacost.