It depends on if the sour crop is caused by an impacted crop that has gone sour, which will result in either a hard mass in the crop that smells bad inside the mouth or a hard mass surrounded by softness. Or, the sour crop is caused by a bacterial imbalance which will result in a swollen, soft crop.
Make sure you really have a crop probably by checking the bird first thing in the morning or withholding food long enough to determine that the crop has in fact not emptied properly.
If it is an impaction you have to find a way to clear it. I would start by isolating the bird and stop feeding any solid foods. Feed olive oil and give massages from the top of the crop downward to try to loosen the mass and get it to pass. If the mass is too large for this to work surgery will be needed. Some people go to the vet and some people (myself included) have done it at home.
If it is a bacteria imbalance you get to again isolated and feed a very controlled diet that will help correct this imbalance. I have not personally dealt with this so I do not have a strong recommendation but have seen people use yogurt or monistat successful on the forums. I would search the threads as there are several good ones.
Do you currently have a problem?