I assume you’re burning MP3s as “data files” to a CD and your car stereo can play them? You’re not making an audio CD, right?
Run the files through [u]MP3Gain[/u].
MP3Gain uses the [u]ReplayGain[/u] loudness algorithm and it changes the volume of the MP3 file without decompressing and re-compressing the MP3 data.
NOTE - Most commercial songs are already normalized (maximized), including many quiet-sounding songs. That means you cannot match volume by making quiet songs louder. Loud songs must be made quieter, and you may notice that most of your library is quieter after using MP3Gain.
Is there a way within the Audacity program to “equalize” the volume level amongst the songs I have?
There is a [u]ReplayGain plug-in[/u] for Audacity, but if you start with an MP3, the MP3 is decompressed when Audacity opens it, and when you re-export to MP3 you’re going through a 2nd lossy compression process.