A murmur may be indicative of aortic stenosis, but it may be indicative of other conditions and it may not be indicative of anything. Did the vets say what grade murmurs they heard? A grade 1-2 is considered mild and in a puppy, may be outgrown - anything 3 or above I'd take to a cardiologist. At 14 months, I'd probably see a cardiologist regardless of grade - they can give a more accurate grading and if necessary do a Doppler for a definitive diagnosis. (It may be a good idea to repeat at or after 24 months, as well, as AS can progress through that age.)
If it is AS, then the degree really is an important factor in prognosis. Mildly affected dogs tend to live normal lives; most moderately affected dogs do as well, though some exercise restriction might be needed, especially as they get older (no long runs or games of fetch in hot weather, etc.) - sometimes these dogs will develop some heart failure as they get older, as well, but generally not to the extent that it would be a solitary cause of death. Severe cases generally have a poor prognosis and shortened lifespan.
