This sounds like you were away for some time, so you have to "rejoin the ensemble". In English instead of efforts we give only 1 effort. I know that this may sound weird. You can also say many efforts and be grammatically correct, but it sounds unnatural. The concert is an event, and while you are "in" it, your performance is "at" it. At the end you need to determine how precise you want to be, either "an instrument" or "the flute". Both of these options are correct, "the instrument" is only talking about 1 very specific instrument, like これ or それ in Japanese.