will do it automatically.
There is also ReplayGain and Apple Sound Check which work in conjunction with the player-software to adjust the volume at playback-time, but your car-player won't support those.
Or if you don't have to match the volume of your entire music library, you can do 20 tracks manually -
1. Normalize (maximize) all of the tracks with the Amplify or Normalize effect. Amplify will pre-scan your file and adjust up (or down) if needed for maximized 0dB peaks. After normalizing all of the songs are as loud as they can be without clipping
(distorting), but they may not sound equally loud.
2. Listen to the tracks and if the volumes are not equal, choose the quietest one as your reference.
3. Apply Amplify, adjusting by ear to bring the louder tracks down to match the reference.
Most commercial music is already normalized (maximized), including "quiet sounding" songs. Any
volume-matching will usually result in lower-overall volume
(which hopefully you can make-up for with analog gain).
mp3 music tracks
As you may know, MP3 is lossy compression.
If you open an MP3 in Audacity or any "normal" audio editor it gets de-compressed. If you then re-save it (export) to MP3, you are going through a 2nd generation of lossy compression
and you may notice quality loss. If you have the CD or other lossless original, do all of your editing with a lossless format, and compress to MP3 once as the last step. If you don't have a lossless original, try to minimize the number of compression/decompression cycles, and it might
help to export to a higher-quality (higher bitrate) MP3.
There is a special purpose MP3 editor called mp3Directcut
that can edit MP3s without decompressing. MP3Gain also adjusts the volume without decompressing/recompressing, and ReplayGain & Sound Check don't "touch" the audio data at all. Without decompressing the MP3, you can only adjust the volume in 1.5dB steps, so the adjustments are not as precise (so that's the case with MP3Gain and mp3DirectCut), but usually that's good enough.
* MP3s are not actually hard-limited to 0dB like CDs or WAV files, but your digital-to-analog converter is. In fact, since lossy MP3 compression changes the wave shape, some peaks get a little higher and some a little lower so the conversion to MP3 often pushes some peaks over 0dB.