Here's my 'tips and tricks' to being able to hear better.
*I* found that when I was going through nursing classes, the sound wasn't as muffled as what I thought it was ... I was just getting a lot of feedback from stethoscope movement and the cuff when it was deflating. Here are some suggestions:
Pick a partner that is skinnier. It's easier for cuff placement. Larger individuals tend to have weird shaped arms and it makes placement of the cuff more difficult.
Put the cuff as high up towards the persons axilla as possible. The purpose of doing this is so that when you are auscultating for the systolic and diastolic your cuff and cuff cords are not touching the stethoscope.
When you're holding the stethoscope bell, keep your fingers apart and NOT TOUCHING each other. Also only hold onto the rubber portion. This will minimize the noise from your fingers moving on the scope.
Try a couple different things, but sometimes it's easier to hear higher blood pressures and it's also easier to hear it when the heart rate is faster. If they'll allow it, make your partner do a few laps around the parking lot to get their rate up. If they won't, then do this next step.
There's nothing saying that you can't take your time. It's class, so you typically get a few attempts. If the person is at rest, the blood pressure shouldn't vary much. Take the blood pressure SLOWLY... only a few mmHg at a time. Listen intently for your systolic (first few wooshes... it's subtle at first). Once you hear your systolic, make note of that number, and pump the cuff up 20mmHg higher and repeat. If you get the same number, then you're golden. If you don't, then try it again. Doing this will make sure you get the right number. Go a bit quicker till the sound disappears, which is your diastolic. It, for me, is a bit more obvious than systolic sometimes. Once the sound disappears, try the above (increase pressure to 20mmHg or just an estimated few pumps) again and make sure you get the same number. THIS way you will look like you've done it before, AND it also makes the teacher know you're double checking your work.
When *I* did it I had to be within 4mmHg of what we got and the teacher got... 2mmHg on either side, and I did well. The importance to me is eliminating every noise you possibly can get.
Oh and don't forget to take the pulse before you do all this so you don't forget to when you get your BP correct!