snd_prefs_s7 is written by the preferences dialog -- I don't think it is overwritten anywhere else. The Snd initialization file is "~/snd_s7", described in grfsnd.html; it is the place to add functions you want every time you run Snd. If you want to add that line to snd_prefs_s7, look for save_prefs in snd-prefs.c.