MP3Gain Express has been updated to version 1.1.1, with a critical bug fix and some new features. Some users reported file corruption caused by the previous version. This only happened when the path to the files processed (folder and file names) had specific characteristics, such as long folder names with several spaces. This is why when someone sent me a file that didn't work for them it worked fine for me because I would usually put the files on my desktop. I never ran into the problem myself because I usually kept my own music in folders with short names.
New features include automatic clipping prevention, subfolder processing, and the ability to undo which restores previously processed files to the original volume. It also fixes a problem with automatic update, so this version needs to be manually installed. From this release onward, automatic update should work correctly.
You can download the new version here. When I first released MP3Gain Express, I wasn't expecting to ever add some of these features. But it turns out that it's the most popular project I've ever worked on, so I will continue to revisit feature requests every once and awhile.