tasya vachakah pranavah
Tasya: His (of that ishvar)
Vachakah: Description, name, calling
That Ishvar's name (dhwani) is om.
Om is the essence of Creation, the primordial sound, which is constant and has no beginning or end, from which everything emerges and to which everything returns. That Om is the frequency which corresponds to ishvar, the route to ishvar.