Unix pipes and positional arguments
Many Unix command-line utilities use "-" as a shorthand to mean "don't use a real file here, use stdin (or stdout) instead". Sox is one of these programs:
This is from the sox manpage
SoX can be used in simple pipeline operations by using the special filename '-' which, if used in place of an input filename, will cause SoX will read audio data from 'standard input' (stdin), and which, if used in place of the output filename, will cause SoX will send audio data to 'standard output' (stdout). Note that when using this option, the file-type (see -t below) must also be given.