Need some extra confirmation on this one, but as far as I can tell, the Riftlocker is using the same sound effects as the Phantamos.
I am almost certain that pre-Winterfest it used a different set of sound effects that sounded very much like the Obsidian Carbine.
A bit of crawling into the file system shows there are indeed 2 different sound effect sets for the Riftlocker and Phantamos but for whatever reason I'm only hearing the Phantamos set for both guns.
I noticed an issue with the 4* and 5* tundruses too
the tundrus makes an icelike cracking sound, but the mega tundrus and winter grave dont