Hi, valtaro,
To lower the volume of a segment, place your cursor on the clip and, and while holding down the r key, press-drag over the region where you want to lower the volume. As you drag, a yellow box will appear outlining the region you chose. After you have outlined the region place your cursor on the horizontal volume line and pull it down to lower the volume in the box. You can also slide the little yellow diamond markers around to vary the sound.
Another way to lower the volume of only a segment is to place your cursor on the horizontal volume line until it morphs into a 4-way cursor and, while holding down the Option key, click on the horizontal volume line to place yellow diamond markers side by side at the start or the segment, and do the same at the end of the segment. Then pull down the horizontal volume line between the two sets of markers to lower the volume between the in and out points.
-- Ric