I felt my heel [squish downward into mud]

The protagonist now lugs a wounded woman who rescued him from a forest fire.

I removed my clock where it was tied about my waist and I spread it out beside her. I inched her onto it as carefully as I could, folded it over her to protect her against flames and began dragging her in what I hoped was the proper direction.
We made it through a shifting patch work of fire and smoke. My throat was raw, my eyes watering steadily and my trousers on fire when I took a big step backward and felt my heel squish downward into mud.
["Trumps of Doom" of The Great Book of Amber by Roger Zelazny]
I'd like to know if "squish" means to make a soft splashing noise, not squash here.
Thank you in advance for your help.