No, the strings are used for chordal support, but the piano has individual notes that need to be heard. I'd say, first, turn down the piano reverb and make it louder. Next,
for the flute, instead of making it LOUD just make it the same volume as the saxophone. It's at a higher FREQ than the sax, so there'll be no blending/clashing whatsoever and it's okay to make them the same volume.
Also, that just means the volume is getting too loud so that your sound card can't handle the output so it starts to clip. I'd recommend lowering the volume of the master channel until it stops.
If the frequency meters turn yellow and red (at the top, should be where you see the frequency spectrum display) , that means you're exceeding a certain threshold so it starts to get iffy. For full details, I recommend Zircon's production guide.
It's an amazing article and teaches you almost everything you need to know about good production values. It's no substitute for practice, but it's a great place to start practicing. xD It taught me all about mastering and such.