It's with mild surprise that Serph observes the song seems to work for V even though he's not Sera and V isn't a demon, so there is no inherently calming property to Serph's voice. Maybe... in addition to the strange powers that Sera had, this is the other reason she was always singing for them? That the soothing and calming nature of singing wasn't because it was from Sera, but partially from the act itself?

Mentally, he notes down this new revelation.

So... even if he isn't totally comfortable with even just humming, Serph continues to do it for V until it seems like they no longer need the song.