Strangely enough, together with the repeated emphasis on setting bounds around the mountain and the prohibition of touching it, God simultaneously tells Moshe that when the yovel sounds, the people may ascend. This command begs clarification.
All medieval commentators understand yovel to mean the sounding of a shofar.
The problem with God's command concerning the shofar blast relates first and foremost to its content. Throughout this shofar blast, which is becoming increasingly louder and stronger, the Shekhina is upon the mountain. Then and only then will it be permissible to ascend the mountain. This interpretation makes perfect sense in our context, since Moshe is commanded to set bounds for as long as the Shekhina is upon the mountain. The clearest demarcation of this period is the sound of the shofar while the Shekhina descends.