Hi everyone,
I received a very clear answer from BIAB , my problemis solved.
Here is the answer..could be useful for some with the same problem:

I see what's causing your problem. You're turning the Piano and Bass track volumes up all the way, and you also have the "All Songs" volume slider turned all the way up on the mixer, and really should turn everything back down to a more reasonable level.

Bear in mind that digital audio can only be turned up so far, and if its level exceeds 0 db, you'll get distortion. So we have a limiter always running on the master output channel, to stop you from turning anything up far enough to distort the signal.

A limiter is a kind of compressor, and the way it works is that when a signal's level gets too high, the limiter turns everything down to compensate. So you already had a piano playing that was far too loud, as it had been turned up to the max in two places, and then you turned the bass up all the way, which made things worse and forced the limiter to turn things down even more.

If Band-in-a-Box's output is too quiet, you should simply turn up your speakers.