I'm not sure if this would bother me or not. It depends on what mood I'm in. Some posts have indicated that people talking loudly, laughing, children being loud is annoying. Although, I would agree that this type of behaviour is sometimes annoying it is to be expected as this is what most people do, talk, laugh, etc. But, playing a guitar, radio, or any type of music is not a normal everyday occurence on a hotel balcony.
I really think that one complaint to the front desk would stop your husband from playing even if it was being done softly. I agree with another post that said, if you husband asked if the music was bothering me, I would probably smile and say no, it's fine. Even if I was not enjoying it. I wouldn't want to embarrass him or myself. A lot of people are very uncomfortable being put on the spot.
I think its pretty obvious that the hotel would ask your husband to stop playing even if only one person complained. I don't think they would want to have to move other guests around.