It is a text file, with one line per song. On each line, there is a string enclosed in quotes like this:
"I am Negamask!"
The string on the first line is the name of the first song. The next is the name of the second song... This continues until you have read a line for each song.
There are always 100 records in SNG. Blank records are simply a line containing
Nameless songs can not be used in versions of Custom using this lump, even if the BAM file exists in the game file.