Basically, this just adds a dolphin chirping(?) sound to Marth's up special. It is called DOLPHIN Slash after all, and if Falcon Punch can make a Falcon noise, Dolphin Slash can make a Dolphin noise.

In case you already have a voice mod on Marth and just want to add the dolphin sound, download the .wav here: https://www.dropbox.com/s/7zbbw8n2plneyls/dolphin%20slash.wav
and replace Audio[34] under Marth's [219] RWSD (51269)