Hear, O my people, and I will testify unto thee: O Israel, if thou wilt hearken unto me;
Hear, O my people, and I will testify unto thee - answering to Psalms 50:7. The fact that ye are "my people," called into being, redeemed, and sustained by me, is the strongest reason why ye should "hear."
O Israel, if thou wilt hearken unto me - thou shalt be blessed (Psalms 81:13-16). An ellipsis often occurs after "if" (cf. Exodus 32:32; Luke 19:42).