Playing vintage mp3 files

Assume you have a computer.

See -> Set up iCloud Drive -> Set up iCloud Drive - Apple Support

Transfer the mp3 files from the external drives to iCloud Drive.

If you have a lot of mp3 files, put them in folders, for example per album.

Tap Apple Files app on the iPhone -> tap iCloud Drive -> scroll to & tap on an album & play a mp3 file.

There are several apps that can play Mp3. I am not aware of a "Music Player only" - but the VLC app can play everything, including Mp3. VLC can be downloaded for free in the App Store.

Once VLC downloaded, open the app and tap "Network" at the bottom. Enable Sharing via Wi-Fi. Two links will appear, use one of them in your browser on your computer, for example https://iphone.local

You will now see this page in your computer browser:

Just "Drag n Drop" your Mp3 files into the app. Your music will now be found and played in VLC on your iPhone.