I was curious about this too, so I decide to try it out.
I tried replacing the sound that plays when you cast the Razorblade Typhoon spell but all that did was mute the casting sound.
I made a simple texture pack folder with the folder structure: "Content/Sounds"
I downloaded TConvert and converted a 1 second long .mp3 sound effect and converted it to .xnb and placed inside /Sounds, replacing Item_84.xnb.
Clearly something happened since the sound is mute now but maybe I need to do more than just convert it so the sound so it actually plays properly, but who knows.
Funny enough, replacing the sound directly works just fine.