In a previous post I spoke about the peculiar language in Galatians and Colossians that describes the Old Covenant being mediated by "elemental spirits". The Epistle to the Hebrews teaches the same, but describes these agents as "angels". In all three epistles the purpose of highlighting the angelic administration of the Old Covenant is to reveal the superiority of the New Covenant that is mediated by a Firstborn Son (Christ) who allows us to participate in divine sonship (not under the tutelage of angelic tutors).
The elemental spirits or angels could refer to the theophany, or more accurately the "angelophony", that occurred at Sinai. The lightning, thunder, fire, etc. are "elemental". This might be the case.
I would like to make another suggestion. The "mediating angels" is a reference to the the two angel statues that stand on either side of the ark of the covenant "mediating" the presence of God to the High Priest, priests, and tribes of Israel. There were also cherubim woven into the veils of the Temple. They refer back to the cherubim that kept Adam and Eve out of the Garden of Eden. Hence, these angels "mediate" God's presence between God and man. The Old Law centralized at the tabernacle was a microcosm guarded and mediated by images of angels.
Christ tears this angelic veil by his death and thus reveals that His covenant is superior. The human does not need to worry about sneaking past the sword-bearing angels in order to return to the divine presence. He enters the presence through Christ.
Incidentally, this view compliments St. Paul's understanding that the Jewish "elemental spirits" are essentially no better than the idolatrous gods of the Gentiles (see this post). In both Old Covenant Judaism and Gentile Idolatry, religion is mediated through material or "elemental" shrines inhabited or protected by images or statues. The Old Covenant Temple was sacred and good version of the blasphemous versions of the Gentiles (the latter of which was inhabited and guarded by evil fallen angels, according to St. Paul). Yet Paul and the author of Hebrews make the point that both the Levitical covenant and Gentile paganism lack the notion of sonship or filiation that only Christ can mediate to us by His own divine Sonship.