I've been looking for years. Short answer: there aren't any.
Very few with sinking as the primary focus. Wasn't a fan of MzD's quicksand lunchbox file, h4yl33's quicksand. mistress jo4nn3's not-so-quicksand. E5uccubu5 has the relaxing sands file but honestly I think there was a better sinking part in another file, I think it was 'deepener and amnesiagasm'?
Very small part in IV's skeptica1 s1ave, also failed to scratch the itch.
There are some audioplays on gonewildaudio, most written by sandystocking2, nothing spectacular. Maybe throw an induction in-front of one of them, or one of those "believe you're in the story" type files.
Ultra's Hypnotic Quicksand Bondage
Sh1bby has some light sinking into latex stuff, as does E5uccubu5. one of the better (low bar) sinking files I've heard was wizard hypnosis latex ocean obedience and pleasure training but its male voice and the dude was heavy cult/creep vibes. Not for everyone.
Consumed by quicksand - hypnomatic hypnosis on youtube if you're desperate. Or "Mud Induction Hypnosis" by miles