Hello Doru.

I don't have a complete list of onomatopoeic words, but you can find a short list here:

http://teenwriting.about.com/library/blank/blstory1459.htm?once=true&

It seems to me that you want something like "squish" for the sound of high heels sinking into melting asphalt. Don't be shy about making up a word that seems to sound like the effect the author intends.