If you need to apply some filter to a group of mp3 files at once, e.g. to add treble and then save all of them, without changing the original files (just in case something goes wrong), Audacity can help! As is known, Audacity is not an exemplar of simplicity and user-friendliness, but, if you read carefully you won’t have any problem doing this batch processing of your mp3 files.

At the File menu of Audacity you will find the “Edit Chains” command (in recent versions go to Tools / Macros); Add a new chain (macro) naming it e.g. MyMp3Fix. At the right pane you will need to add the filter to apply to your files. In our case, we add some treble. Double click the first row and select the Equalization command. Double click the command and edit the Parameters, configuring how much treble you need to add. If you choose the graphic equalizer, you can easily increase the upper frequencies, from 12 to 20 khz. Adding 3 db normally would be enough to give some clarity to your files, without distortion.

After you finish with this command, add a new one (double click the next row) and choose the “exportmp3” option, double click on it and press OK to add it to your Chain. Save your new chain and go to the File menu. At the File menu, go to the Apply Chain option, and press the “Apply to Files” button. Locate the folder of your files, select those you need to change, and press the OK button.

Unfortunately Audacity won’t let you choose directory to save your files. By default it will save the changed files to a new folder inside the folder of the originals. That’s it!