The posuk says “He called to Moshe…”. Rashi says, the voice traveled and reached Moshe’s ears, however, all of [Bnei] Yisrael did not hear. Rashi teaches us that the voice that reached Moshe’s ears was very loud (see Rashi ‘Me’Ohel Moed’). Why was it necessary for the voice to be so loud if it couldn’t be heard by Klal Yisrael anyway?
I heard (indirectly) from Reb Moshe Feinstein that the purpose of this Heavenly voice was that one should feel that he was being commanded directly by Hashem Himself even though we were actually taught the mitzvah through Moshe. Since the voice that reached Moshe was projected throughout the entire world, it was in truth, directed at us. The reason why we did not hear it was due to our own limitations. If we would have been worthy, we would have heard it as well.
Which teaches us that Hashem’s command is more stringent than if only Moshe was instructed to tell us.