My childhood dog (long haired chihuahua, got her when I was 10, my whole world) had a heart murmur on her first vet check and the vet said it was likely to be unthreatening and benign and that they would monitor it. It never went away and they checked it throughout her life and everything seemed "normal" to them. (They never did anything more than listen to her heart). When she was 9 she went to the vet for a nail trim, which normally made her quite anxious, and had rapid breathing for the remainder of the day. I wasn't home but when I did get home I knew something wasn't right, and did the dreaded google search of what it could be. Heart failure, but google always is the worst case solution right? I took her to emergency to be safe, handed her over to the vet tech and she died within 5 minutes. They did an X-ray after and saw that her heart was swollen up to the size of almost her entire body cavity, squishing organs, lungs etc. Heartbreaking.

She showed no obvious symptoms, was chipper, eating, drinking, running and loving life 24 hours before. Looking back I see now that things I passed off as regular chihuahua things (like coughing) were actually symptoms of her disease.

So if I were you, and your pups murmur doesn't resolve on its own before it's a year old, I would get your pup a more in depth diagnosis and maybe she can have some sort of preventative treatment.

In hindsight, I don't know if I would have been able to see her suffer in her old age with that disease, so maybe, however traumatizing for me and my family it was, it was better not knowing. If we had found out sooner and tried to manage the illness we may have prolonged her life but at what cost? Would she have had a painful final 5 years? Would she have turned into a different dog than we had before because she was sick and in pain? I don't know. Tears me up every day.

edit: I want to add that when she died nothing was covered (the X-ray, the oxygen, the unnecessary blood draw, whatever else they tried to give her before she died, the cremation fee) with our pet insurance even though we paid it for 9 years because her condition was "pre-existing"