Normalize Music Volume with Audacity
Audacity is a free and multi-platform audio editor that you can normalize audio volume with it. The steps are as follows:
Step 1. Open Audacity, click "Open" under the File tab, and then hold Ctrl to select the audio files you want to import, or simply drag and drop all the audio files to the main interface.
Step 2. Press "Ctrl + A" to select all imported audio files, click "Effect" on the menu bar, and then select "Normalize".
Step 3. Check Normalize maximum amplitude to, type the desired volume level into the "dB" box. Click OK to start to normalize audio.
Step 4. After done, click "File" again and select "Export Multiple". Finally, set export format and location, and hit Export button to start saving.