Some DOS trackers allow ASCII control codes to be used in song messages. For example, consider this 669 module: https://ftp.modland.com/pub/modules/Composer%20669/Buttwheet/the%20ritual!.669

Several ASCII codes below 32 are used in the song message, using the standard CP437 characters assigned to those codes (see attached screenshot). libopenmpt should probably transcibe those control characters to normal printable UTF-8 characters. Full support for control characters 10 and 13 may not be possible until full unicode string support is implemented for CSoundFile, as the conversion to printable unicode characters would have to happen before the three-line fixed length message field in 669 files is converted into a multi-line string.