Perish Song and Sing already work on Ghost types. It's just Thunder Wave that obeys type immunities like this, and that's not a sound move.

Liquid Voice really is in the same bucket as abilities like Friend Guard and Symbiosis right now, the only actual advantages it has over Torrent are exclusive to Doubles. No Singles specialist will care about Water Hyper Voice not harming teammates, nor would they be crazy about more stops to Perish Song beyond the usual Soundproof.

I will say that I'm interested in seeing anyone try out Water Echoed Voice within casual runs of SM/USUM when Liquid Voice is released though. Super Mystery Dungeon did give me a mild soft spot for that move.