With Audacity we can easily batch process files to apply conversions or effects to many files in a list. To do so we first have to define a “Chain” containing the effects we want to apply.

This is done with “File –> Edit Chains…”. In the now opening window press the Add button on the bottom left to insert a new chain (give it a sensible name):

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Then choose the effect and it’s parameters to insert to the chain (here shown for default values and the Normalize effect).

Important: we always need to also add the effect “Export MP3” (or any other export format) for saving the resulting conversion to disk.

When done leave this window with OK to open “File –> Apply Chain…”. Select the Chain we have just created and load all files you need with “Apply to files…”. Several files can be selected from the file chooser that opens.

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Processed files will be saved in a new subdirectory “cleaned” in the original’s path.

Source: https://askubuntu.com/questions/246242/how-to-normalize-sound-in-mp3-files