Voice fetish, eh? I've never wanted a doujin to be narrated until now, just so I can hear the voice that can turn straight friends queer and make busty teachers drop trou.
Looks like it's time to cancel my gym membership and use that money to buy the Sound Like Barry White instructional DVDs I saw advertised last night.
And i thought scat was wierd (and gross) enough, now shes horny from his voice? Im dissapointed he didint sing during fucking, that would have broken her mind! XD
Since it's just a hentai, it's just a voice fetish.
While I have heard of a voice fetish before, it could also be a case of Synesthesia (neurologically-based condition in which stimulation of one sensory organ triggers the stimulation of another sensory organ, sort of like natural LSD {you could potentially feel colors}). Still, in order for this to work, she would have to have her hearing stimulate her touch sense. Not only that but it has to specifically stimulate her sex and not just any sound but only this kid's set of frequencies and timbre. So yeah... She just has a voice fetish, now that's just silly.
This is hardly any new fetish. This was already in Wanyan Aguda's Boy Soprano (Chapter 11 to be exact) and as Gram91 already said, the protagonist of the non-h manga Ai Kora also has this fetish.