I am guessing these are mp3 files using ASCII encoding. MB uses the windows default codepage for decoding the ascii text, although the official ID3 encoding is ISO-8859-1 - from the early days of MB, I just found in many cases the windows default codepage works better for most people.
If you use MB for tagging, it uses UTF-16 encoding which prevents this type of issue from occurring.
If its an option in your tagger, use UTF-16 (eg. mp3Tag has that option)