I'd recommend having an echocardiogram done by a cardiologist to see what exactly the nature of the heart murmur is. There are medications (beta-blockers, calcium channel blockers, etc) that can help, but you need to know what's causing the murmur first and go from there. Many cats live long happy lives with a murmur (my cat being one), so I wouldn't panic yet. She's a beauty, by the way!