The audio, obtained by TMZ, could be heard over LASD airwaves this past weekend, and we're told it came from a rookie female deputy working at the Men's Central Jail that night. It sounds like, during a break in her shift, she got into some risky business.

Take a listen ... you can hear a woman giggling and warning her partner that he's going to rip her underwear, to which he responds playfully.

Now, law enforcement sources tell us this deputy went out to a parking structure that night, and her bosses believe she accidentally engaged her radio while getting into it. She eventually realized the microphone was open and shut it down, but, as you can hear, the damage was done.

LASD tells TMZ the agency has launched an internal investigation to find out more about the incident -- not to mention whether the man heard on the audio was also a member of the force ... and if he, too, was on the clock. We're told potential punishments could range from a suspension to a firing.

This isn't the first time LASD deputies have been caught doing the deed on duty -- you'll recall ... something similar happened 2 years ago at Universal Studios, and that was also caught in a radio broadcast.