i would like to have some help about the red horizontal line on the audio track.. should it stay as imported (zero Db) or in the middle (which is -6Db) to prevent distortions/filters to the correct original audio.. I am afraid that leaving it as it is imported (which is always zero Db) it is too loud and also the wave graph show me as i am cutting part of the LOW audio.. does it act like a low/high pass filter like this? am i wrong? listening to the final audio i think i should better raise volume with some filter (audio gain filter maybe?)
The level of the clip has nothing to do with hi pass or low pass filtering. The level should be adjusted to suit the balance required and is entirely dependent on the level on the clip as recorded. If you leave it at zero it just means the level is the same as recorded and will not clip unless the original recording is clipped.
If that is the case then lowering the clip will not correct the clipping as recorded. However it is still a good idea as mixing other sounds with something already peaking to maximum will further distort. But the answer to your question depends entirely on the actual clip levels. Just remember that digital audio clips at zero so never go too close to zero. Final mix levels are entirely dependent on where you video is to be shown. Broadcast levels are particularly specified. The web is more cowboy.
Digital audio stops at zero dbfs. So anything louder must clip and distort. Transient signals can be really fast and so your meters might not accurately represent tiny short transient signals that clip.
So most people make sure their metered signal doesn't get to close to the magic zero in case they are having some clipping that isn't being metered accurately. Final Cut Pro meters are not considered accurate mastering meters so make sure your peak signal is a few db under zero (-3dbfs is a common target) just in case.
If you are providing programs to broadcasters they will not accept peak signals over -8dbfs. So you final decisions about levels depends on what you are making and the method of distribution or display.