Christianity's, or at least Catholicism's answer to this, I believe, would be the Pentecost, which involved, if I remember correctly, the Holy Spirit descending upon the Apostles while they were preaching to thousands of people, which caused an apparition of tongues of flame to appear above their heads, and all present to hear the preaching of the Apostles in the language they knew best. I could be wrong about some of the details here, somebody who went to Catholic School would probably know best.

Speaking as another lapsed Jew, I'd have to say that a better general refutation for this argument would be the Empirical argument against (historical) miracles, first articulated clearly by David Hume. To paraphrase, all of the people who were reputed to have heard God speak to them at Sinai are dead now; hell, everybody that knew them is dead now too. Even assuming that all of them actually believed that God spoke to them, which we have no way of knowing with any certainty, since we all have to go on are historical documents which could have been falsified at a number of points between then and now, we have no way of knowing, objectively, whether they did or not. Since their testimony flies in the face of what is generally considered to be true of the natural world (this is why it's considered a miracle in the first place!), we have their word against the entire observable universe. Given their fallible human nature, the universe wins.