The automated machines aren't always available, and even when they are, they are less accurate and provide less information than a correctly taken manual blood pressure. They are of course a lot more convenient and usually provide a great approximation.
There is no substitute for taking the blood pressure manually. It is not a hard skill to learn, and it gets better with practice. There is no excuse, you need to find someone to practice with, if you are going to be a medical profession this is a skill you will need.
If you are having trouble distinguishing the sound of your patient's heart beat remember you also have the dial to watch, it will start to move with the patients heart rate while the pressure is between the systolic and diastolic blood pressures. You can't use that directly for your measurements but it's a good clue that will help you pick up the sound as you learn.