One year on Shavuos, Rav Shloimke of Zhvil zy”a told his shamash, Rav Eliyahu Roth zt”l, that a bas kol (Heavenly voice) rings out every year before Shavuos and asks Klal Yisroel: “Are you ready to receive the Torah?” This is as the pasuk says (Shemos 19:5): “If you will surely hear My voice and observe My covenant and be to Me as a chosen nation.”
He then told him, “The bas kol just rang out. Therefore, we should shout together; ‘Naaseh V’Nishmah!’”