At first, you have to open the MP3 or other audio or video file you want to adjust its volume. To do that press on the "Open" button of the toolbar or select "Open" from the "File" menu.
You can also drag and drop the audio or video file to open it.
Secondly, mark which part of the MP3 file you want to change the volume.
You can do this by selecting the part of the MP3 file's waveform.
Alternatively, if you know the start and end time or duration of the desired part, enter the start time, end time or duration in the appropriate text boxes.
Now press on the "Adjust Volume" button of the toolbar.
Alternatively, you can select "Adjust Volume of Selection" from the "Effects" menu.
Then a new screen will be shown where you can choose to set the volume of the selected part to a percentage of its initial volume or to increase or decrease the volume by a specified value of decibels. To decrease the volume enter a negative value in the textbox and to increase it enter a positive value.
Press on the "Output Format" button to specify various output format settings or leave the default ones.
Lastly, select "Save Whole Clip" from the "Cut" menu to get the result file.
In case, you want only a part of the MP3 file, after selecting it, you can choose also "Cut Only Selection" from the "Cut" menu.