I don't see why you'd want to mix alcohol and propranolol. I don't see why you'd want to take propranolol at all to be honest with you since it has no recreational value.
But if you need it medically, then you should PROBABLY be fine. I was prescribed propranolol for anxiety which I thought was completely useless. Anyway, I came home once wasted on alcohol and cocaine and couldn't sleep at all since I was high as fuck on a shit load of cocaine and MDMA. I took one 40mg propranolol and it slowed my racing heart down to a normal level so I could sleep. I didn't have any problems and slept and woke up fine.
But I mean if you take propranolol because you have abnormally high heart rate and blood pressure then taking propranolol and alcohol might cause difficulties since alcohol lowers your blood pressure anyways. I would honestly just leave the propranolol out. It's not going to give you any milder effects or potentiate the alcohol just cause risks which outweigh any benefits.