How to increase volume on multiple MP3s at once?

There are different types of gain control:

• Peak normalization will search for the part of the file with the highest amplitude, amplify the file to the loudest level possible and then amplify the rest accordingly. It's a pretty "stupid" technology in the sense of not making use of any psychoacoustics.
• Loudness normalization which brings the clip to a perceived loudness. EBU-R 128 is a standard that is used primarily in the broadcasting industry, and using it makes sure that all TV programs, for example, sound equally loud. (Mostly, except for ads maybe.)
• ReplayGain is an algorithm that analyzes the files on a psychoacoustic basis and amplify them accordingly, so they all have the same perceived loudness.

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Using ffmpeg-normalize

(Disclaimer: I am the author of the program)

You can install a program called ffmpeg-normalize; all you need is Python and ffmpeg installed, then:

pip install ffmpeg-normalize

Now you can run:

ffmpeg-normalize *.mp3 -c:a libmp3lame -b:a 320k

This will loudness-normalize all MP3 files to a given target, and write them to the normalized folder.

I do not recommend doing this for your music collection though, as with MP3s you would have to re-encode all files and may lose quality in the process. Rather apply ReplayGain (see below).

Batch-Normalization with Audacity

If you want to normalize using the Audacity feature, you can do so using the Macro functionality. You can find an example Macro on this page.

Similarly, I would not recommend doing this if you could use ReplayGain.

ReplayGain

I assume you are on Windows, but this works for OS X as well:

• MP3Gain for Windows
• MacMP3Gain for OS X
• There's also a GUI for Linux

What does it do?

MP3Gain analyzes and adjusts mp3 files so that they have the same volume.

MP3Gain does not just do peak normalization, as many normalizers do. Instead, it does some statistical analysis to determine how loud the file actually sounds to the human ear.

It will analyze your files and make them as loud as possible, thereby equalizing all of them in terms of loudness. So normally, it should be enough to let this run on your library. MP3Gain includes a batch option for your convenience. They will not be re-encoded during this process and therefore not lose quality.

However, your car audio would need to support it. Try it out and let me know if it's not working.