You probably cannot convert MIDI audio (.mid, .midi) to PostScript files (.ps) with some converter, but there may be ways how to achieve this conversion indirectly.
First you need to convert your MIDI sounds to a music notation (like .abc files) and then proceed export that notation to PostScript.
The excellent bundle of utilities called abcMID contains all the tools needed for this particular conversion, mid2abc program for generating a notation and yabs program for export that notation to Postscript.
However, several other formats use mid extension for their files, so perhaps you were looking for another type of mid to ps conversion instead. If so, please write in the discussion below what you are looking for, with as much relevant information as possible.