Did the vet do any skin scrapes?
Does your cat go outside at all?
Are the sores on the ouside part of the ear or down inside the ear canal?
In that area it might be infection, skin/ear mites or it could be from scratching and missing the itchy spot.
As the initial meds did not clear this up you may need to have a sensitivity test done to check to see what meds the infection might respond to.
If the area was not checked for ringworm fungal infection that is one possibility.
http://www.marvistavet.com/html/body_ringworm.html
ears are a common spot for that to show up.
You may want to check for flea dirt - my cat came home from my vet with a flea and it multiplied! When cats try to itch their head area sometimes they 'miss' and itch the closest possible spot leading to sores from the skin getting broken. You can read about fleas here
http://www.pet-informed-veterinary-advice-online.com/flea-pictures.html
If any home care done doesn't resolve this you should start by calling your vet and see if they want to change meds or see your cat again for a closer look into the problem.
Hope this helps you!
One thing you can try for home care is to clean the spots off with Betadine iodine solution and then apply some Neosporin or Triple Antibiotic ointment. None of that will hurt the cat if any manages to be licked and either could help with a surface infection.
You might also see if trimming the rear nails helps to resolve this. This may have started with an itch, that became an infection, and the infection could itch making the cat scratch the area more.
You can read about ear mites here
http://www.marvistavet.com/html/body_ear_mites.html
Cat Health, Behavior, Care Expert
30+ years cat owner, rescue, breeding, study of behavior & health care