Is it anything like when you go down a 45 degree angle slope in this game, and just hit crouch and stop walking, how you keep going? That's implemented in the game, you could possibly use part of the code for that for crouch sliding.
Great Idea btw, + all the way.
CROUCH SLIDE FTW
And for the sound, you could have someone slide something through some gravel to make the sliding noise, it would take at max 5 minutes to get it converted to .wav or the appropriate file type.