It looks like that both .sng and .mls files are used by the same program WorshipCenter Pro and thus sng to mls conversion can most likely be achieved using this program. As far as we know, this is the only sng to mls conversion that makes sense.
Convert WorshipCenter Pro document to WorshipCenter Pro document file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert sng to mls on Windows
A presentation software for Windows
Convert from sng file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.