I know that they aren't supposed to outgrow a murmur after 16 weeks, but I had a different experience. I adopted a boxer fifteen years ago. He had been returned to the breeder because he had a rather highly graded heart murmur. He was almost six months old when I brought him home and I immediately took him to the vet. The vet, who had a crappy bedside manner, told me that he had a grade 4 murmur and I shouldn't waste my money on any vaccinations because he'd be dead before he was a year old. I had him vaccinated anyway. I drove home crying, holding him on my lap and telling him it didn't matter. That I'd love him for as long as he was with me. I was lucky enough to love him for ten years. We had a kiss my ass party every year. I'd had him to the vet numerous times during his last year and a murmur wasn't detected at all. As it turned out he passed from Addisonian crisis. It was an odd diagnosis for an old male dog, but all the vague symptoms he'd been having made sense. Xrays showed his his heart was significantly smaller than it should be, but that wasn't related to the early murmur at all, that was due to the Addison's. I would see a veterinary cardiologist if it were my dog. I should mention that throughout his life other vets did hear a murmur, but is was barely audible. Certainly not the grade 4 that the original two vets heard.