Some heart murmurs do not affect the pet and some do and of course they can go from nothing to something. The EKG can be done awake sometimes, if the pet is very calm, other times it can sometimes be hard to read and they may need to lay on their side. Sometimes it can tell you where the beat is changing. Ultrasounds, are a little pricey, but can tell you a lot of information about how the heart looks, where any leakage and backflow is coming from, etc. X-rays can also tell you if the heart is enlarged of there is fluid around the heart. Both can usually be done with an awake dog, but at the same time they won't want to stress her if she doesn't want to lay down on her side and back.
If she goes into heart failure there are many meds that can really prolong life and keep the pet comfortable- vetmedin, enalapril, and lasix to name a few. Vetmedin can only be used when they are in heart failure. If she still just has a murmur meds may not help or be worth giving and for some kinds of murmurs they are not necessary. We had a cat with one that was new when he was older, ultrasound was done and there was no need for meds at that time. About a year later he started having more problems, not all heart related and dying. He was older and it was expected at the time that he didn't have a ton of time left and he was starting to look older, but still got around and was happy.
Right now it doesn't sound like she is suffering, but watch for change in activity, eating, drinking habits, secluding herself, more coughing. If you see any changes of course talk to your vet and see what your options are now and if you there are changes. Good luck to you and hope she still has several years to go :)