what are .CHK files used for?
The .chk file IS the map itself. The .scm or .scx is just the storage for the .chk map file. Sounds added to the map remain outside the .chk but in the .scm/.scx.
The scm/scx is an MPQ archive, much like Stardat.mpq. It's where everything is stored.
Think of a map (scm/scx) as a compressed file (such as a zip) and the chk as the uncompressed file (such as the txt inside that zip).