Are you absolutely sure that they heard? How long was the loud guy mouthing off - was it just a one off topic of conversation?
I agree with the other poster that you should have told loud man that your friends next door are muslims and that his comments were uncalled for and unwelcome but there's obviously nothing that can be done in that way now.
If you feel sure they heard then, if it were me, I'd send a message in a card. Keep it brief, say something along the lines of you value them as friends and neighbours and that you are mortified about loud man's behaviour. Assure them that his views are not yours.