MP3Gain

I am assuming that the Tape is being made in some form of Tape recorder that takes in the output from a CD player with the CD made from the MP3 files

If this is the case, I would remove the Cd from the process, take the output from the PC/Laptop direct into the Tape recorder this removes one layer of possible degradation, though with a digital to digital transfer PC to CD this is usually minimal so your method is possible

You could use MP3Gain to set the volume of the MP3s to equal as individual tracks or as an album, (the album setting respects the differences in the volume levels between tracks that the Artist/Producer usually use so is best for accurate reproduction but not always for Car use).

The Track level setting levels the volumes to give all the same output, which is ideal for vehicle listening but is not always what the original recording is designed to produce.
The important stage in this type recording is to test the complete tape output folder, before you commit it to disk and then adjust the MP3Gain settings to give the results you need. After doing this for One Tape I would suggest that you record the tape and test it in the vehicle - If you need to you can re-run the MP3s to through MP3Gain to fine tune the results

Once you have your first tape you can if you wish run MP3Gain on your whole library so that any future Tapes should be quickly produced.

Of course to retain output quality of MP3 or CD music you could try connecting a SmartPhone / MP3Player / or Portable CD Player direct to the Tape unit - this could give you many more songs available (volume levelled as above)

Lots of Cheap adaptors available ...
https://www.amazon.co.uk/s?k=MP3+to+...ref=nb_sb_noss

The stated advantage of MP3Gain is that it just changes the internal master value, without needing a re-encoding of the MP3 File with the potential degradation of the file that can result from Multiple repeat encodings

MP3Gain FAQ

I'm not certain of the algorithm used by Audacity but sounds as if it's not the ReplayGain Algorithm it could result in some file degradation each time a file is edited - Think multiple edits of a Jpeg File

A lot of technical utilities such as MP3Gain rely on Dynamic Link Libraries related to things such as Microsoft Visual Basic, or C "libraries", or other Runtime Code

These libraries often evolve over the years and the runtime code is updated to match. although there is runtime code included with Windows 7 this code is contemporary with Windows 7 when released MP3Gain has been around for many years - It may have originated in the days of XP, and was definitely around for Windows 7, but I believe it has not been updated for a number of years whereas the library code has .

Due to the above the program will require an older Code Library to run - when you install and run MP3Gain windows will notice this issue and prompt to download and install the required libraries

anjou2 said:

I was just a little put off installing MP3Gain because of the warning from Win7 that "things might be missing"

There's a full, portable version called MP3GainPortable_1.2.5.paf.exe that doesn't make you install anything, in case you're concerned about messing up your computer or wasting space and effort. It sounds like you got hold of a "light" version that didn't include the Visual Basic package.

Certainly it "installs" here, i.e. it unpacks to the drive of your choice, without issues. I've not tried using it for anything yet apart from verifying that the program opens, but having read about it, it looks like it offers the advantage of automatically levelling out the subjective volumes of a set of recordings, which seems like it would be quicker than using an audio editor. I'd make backup copies of my files before trying it if I were you, though it claims to be nondestructive. And I have heard that there's no automatic gain-levelling program that can get the levels absolutely right, as subjective volume is too - well - subjective for a machine to fully work out.

Incidentally, there's what looks like a new version of it, also portable, going by the filename wxMP3gainPortable_3.7.paf.exe. Again, it unpacks and seems to run fine as a standalone.

Update: I just tried using Mp3Gain Portable on a real job (a folder of mp3s of differing volumes) and it evened them out pretty well. It even alerted me to the fact that 2 of the original mp3s were already clipped, by listing them in red right from the start. It's pretty intuitive to use. I didn't use the "upgraded" wxMp3Gain because I can't see any useful difference. Download Portable MP3Gain 1.2.5