Does anyone (preferably someone who has played the piece) have any specific advice on how to practise the middle voice of Schubert's third impromptu? At the moment it sounds too loud and bumpy.
There are a few things to consider; here are some:
Make sure you're conceptualizing the top voice as a melody. Play around with only playing the left hand and the melody to make sure you know how you want your melody to sound. Even consider playing the left hand and the middle voice while singing the melody. I'm always amazed at how different things sound once I've taken a few reps to sing one of the voices.
Obviously practice the middle voice alone, striving for fluidity as much as you can. But also practice all combinations of the three voices: bottom with middle, top with middle, bottom with top.
Occasionally record yourself. This can be a difficult process, because invariably you always sound worse than you think. But it's the best way to judge the sound of what you're playing. This will give you the chance to really pinpoint where your weaknesses are, and you can plan your practice time accordingly.
Lastly, the middle voice is just arpeggios. As you warm up before each practice session, make sure to play some arpeggios at a pp dynamic level, trying to make them as fluid as possible. Ideally, you would practice these in keys with lots of "black notes," since that's part of what makes this middle voice so difficult.