Is a heart murmur still a big deal for joining the military?

Depends: Heart murmur is not synonymous with a heart problem. A heart murmur is a sort of "squirting" sound that can be heard when listening to the heart with a stethoscope. Some of these "squirting" sounds are simply the sound of blood being "squirted" out of the heart (innocent murmurs) but they may also be the result of an abnormality of your heart. You should have yourself checked.