My mother has one and is due to the narrowing of her Aortic valve. She's possibly had it for most of her adult life. (There are various different types of Murmur)
She now has Stenosis of the Aortic valve, which is becoming narrower. I think this has been something she has had for a while, but her GP never really told her about it, so now at 82, there isn't much they can do, as she's too frail for valve replacement surgery.
Her condition has worsened over the last 6-12 months and she gets very out of breath, even walking a few feet.