There are very specific herbs to support the heart insofar as its electrical regularity and the strength of the muscle are concerned. Heart problems have been a concern of mankind since time began.
There are other herbs which are specifically useful to reverse any degenerative process. I can make up a herbal extract combination to provide you with your best chance of reversing this tendency which has developed in your dog for whatever reason. The treatment comes as a concentrated herbal extract mix and is given at a dose rate of 10 drops mixed into feed twice daily.
If there is a cough associated with the heart murmur, the cough reflects the fluid which accumulates when the heart is not quite up to strength then you need to support the lungs and heart in order to deal with the accumulation of fluid & ineffectiveness of lungs.
These herbal treatments are safe to use with orthodox meds.
I also recommend that you place your dog on a diet of raw meat and bones i.e. beef/mutton bones with meat on them or raw chicken necks and wings, mince; definitely no processed food and snacks. You can also add some freshly cooked vegetables and rice.
If your dog is not already on this type of diet, then I suggest that you introduce it slowly.