lossy to lossless?

Two possibilities.

One, it is just your imagination. Once data is deleted during the lossy compression process, it is permanently gone.

The other possibility is that the MP3 codec that your player uses directly to convert from MP3 --> playback audio isn't as good as the process that is converting from MP3 --> ALAC. (Or, at playback time, your decoder is doing a better job with ALAC than it does with MP3.) Either way, while no missing data is being restored, one format may allow more to be done with the information that does exist.

This all assumes that you have the playback settings regarding equalization and replay gain set equally for both playback scenarios.

It might be worth engaging a friend to help you with some blind experimenting. Since you have copies of the same tune in both MP3 and ALAC, have your friend play back songs in both formats without you knowing which is which. If you can reliably identify the difference between the two file types you may have found something that can give you better playback.

Note this may be limited to just your system. A different codec or playback program that doesn't struggle directly with MP3s could result in there being no difference between the MP3 file and the ALAC.

Mlsstl,

Thanks for the reply. It probably is my imagination--it has certainly lead me astray in other matters.

And I think you are right about the MP3 codec. If there is a difference, it is probably due to how well the file is converted directly from MP3 to audio. Makes me wonder if I should use a different player for the MP3s.