I finally listened to the tapes closely again (earphones and all). Honestly from the sounds of it, this seems like the closest to what actually happened from what I can hear.
Spoiler-tagging the rest if you're not comfortable talking about the mature content (which is perfectly understandable):
It's very obvious that Paz was being assaulted by one or two soldiers, but it should be noted that Skull Face always seemed to be the one who really moves around in the "scene" which makes me think he didn't do anything to Paz beyond man-handling her (ripping her clothes, grabbing her face/choking her). A lot of the time Skull Face's words denotes an instructional tone towards others in the room("Move." "Show it to him.") which meant that while he wasn't an active participant in the assault, he was the one bossing people around while actively walking between Paz and Chico (he's the only one that's given any "walking" sounds based on his boots).
But yeah, from I could tell during the torture sequences: Paz is already being assaulted just as Chico arrives -> Chico is brought in for questioning -> Skull Face makes threats of violence against the two -> Paz is whipped/beaten due to Chico's defiance -> Chico gets whipped/beaten only for Skull Face to realize he's taking the blows for Paz -> Skull Face has Paz stripped by a soldier to show her scars -> Paz is assaulted by one/two soldiers to get Chico to talk -> Skull Face gives Chico the choice to talk or assault her himself -> Chico talks (hinted that he never assaulted Paz?) -> Paz offers herself to Chico while in captivity (never know what happened here)-> Chico is forced to whip/beat Paz to get her to talk until Skull Face gets tired of it
I should mention that I only did this breakdown for analytical purposes, so I don't end up making assumptions especially when it comes to a touchy subject like this. That said, feel free to correct me on this if anyone thinks I might have skewed something.
That said, Solidus I'll admit, I've actually seen some folks defending the implementation of this on various sites, most of which outright claim that the use of an audio-tape is a smart move on Kojima's part (showing sexual violence without actually "showing" it) so it's not like Gaspar is the only one making any defensive claims about it (at least outside of SSF forums that is). While I honestly have little opinion on the matter (I'm still waiting for Phantom Pain to judge MGSV as a collective whole), I just thought I'd point that out at least.