Cardiomyopathy is completely different to Aortic Stenosis.
http://www.boxerbreedcouncil.co.uk/as.htmAs you see Aortic stenosis affects the valve/s of the heart.
http://www.boxerbreedcouncil.co.uk/cmvetarticles.htmhttp://www.boxerbreedcouncil.co.uk/cmreports.htmCardiomyopathy affects the electrical conduction within the heart causing the heart to beat erratically.
Eventually it affects the muscle of the heart and then eventually causes death or can be sudden death.
To see if the parents are heart tested clear of aortic stenosis you should be able to see them here
http://www.boxerbreedcouncil.co.uk/heartlist.htmThis list of heart tested clear dogs is sorted by dog or bitch and then in Alpha order of the KC registered name.
I have known people that the vets have scared witless with your puppy has a murmer only to be completely surprised when the
puppy comes back for a check up to be told not there now or it's not as serious as they thought.
I've had it myself where a puppy was graded one (which is an acceptable grade and classed as clear for breeding)
and then went on to be graded a zero as an adult. So as you can see it can and does happen.
I guess most in these cases were genuine puppy flow murmers as you can read about in the puppy handout sheet.
My grade ones have been perfectly fine and lived to the expected average age for the breed.
Also ones that have had grade two's that some friends have owned have lived perfectly normal lives.
It's only grade 3 and above that you can have problems.
To be honest if your vet doesn't specialise in cardiology you won't get a correct grading of the murmer.
Hope this helps :)