Well, sfx can often sound "unnatural" even if they have natural sources (I've seen fart clips with obviously real farts that sound nothing like farts when you listen to the sound track alone; a guessing game would give you all kinds of results except fart, and yet there is the video evidence that it was a real one). Plus, note the modulation / echo effects when the fart is actually consumed: the oral cavity is after all a resonance chamber capable of altering sound parameters depending on the form it takes (see e.g. vowel sounds), like a primitive analog audio editor.
While on this topic, I'd like to mention that the scat scenes are a little underwhelming, since what you usually get in Fem-U is the female character saying something like "there it comes!" and the MC dealing with it, without audiovisual cues to suggest that there is more than simple face sitting going on. Mayhaps the quest for the perfect fart could lead to interest in the discovery of the perfect scat? It would be quite charming to see female characters sprinkling the MC's face and then mocking him for it, before proceding to the main course with obvious glee and relief, followed by clear swallowing sounds to further legitimize the situation (and perhaps a few further remarks by the female character on how good or bad he is at it, and whether or not he needs to improve). The MC could then express his thanks to the female character, thus tying everything up with a neat bow.
In fact, the MC in general is not sufficiently appreciative of the lovely treatment he receives from the various girls. The skullgirls are a particularly sad case. Why not have him thank them for, say, kissing their shoes (such a pity that he doesn't even lick the dirty soles), or cleaning their toilet, at least after a few sessions? As his reputation grows, the spreading rumors could lead to other girls also merrily mocking him for these services and even asking him to clean their own room toilets or to kiss (perchance to lick?) their shoes, with him showing due appreciation (more teasing and taunting overall, e.g., from less known girls or even strangers, would also be quite wonderful in the game; why not a few mocking nicknames that become more frequently used as time goes by, or shaming items given by the girls for the MC to wear like a headband or paper hat with insults, or maybe even insulting head tattoos?). The MC could also thank April for demanding proper respect from him, and even, as the MC further works at her store, decide to ask her to donate part, or even all, of his salary to the skullgirls in recognition of their beautiful social justice work and their kindness toward him (April has, after all, connections to the skullgirls).