How to make audio louder
Drop in a file. Normalize to −1 dB finds the loudest moment of the selection and raises everything until that moment sits just under full scale, as loud as it can go without distorting. Or drag Volume anywhere from −30 to +30 dB: +6 dB is about twice the level. Play it to check, then save.
Clipping
Digital sound has a ceiling. Push a recording past it and the loudest moments are cut flat, which sounds harsh and crackly. The line under the slider gives the loudest moment at the volume you set, and warns when it would go over.
Quieter works too
Drag the slider below 0 to turn down a recording that is too loud; −6 dB is about half. Turning it down cannot repair sound that already distorted when it was recorded.
Questions
Why is it still quiet after normalizing?
Normalizing goes by the single loudest moment. A recording with one shout or one bang already has its peak near the top, so there is little room to raise the rest. Cut the bang out with the handles, then normalize again. Making the quiet parts louder than the loud ones is compression, which this tool does not do.
Can it boost more than 30 dB?
Not in one go. Save the file, open the saved copy and raise it again; but a recording that needs that much will bring its hiss up with it.
Does it change the whole file?
It saves the selected part at the new volume. Leave the handles at the ends to keep all of it.