Would it be possible to add Voice Cues when the Ladies respond to you talking to them? Something simple like a Audible hum.
Also if this is possible would adding a small about of emotional tone to each murmur would help the immersion.
Something like;

Mmm? (asking answering a a non sexual question)
Mmm...(Thinking/non committal )
MMNM!?!(Anger/Shock)
MMM! (Pleasure/Desire)

Another possibility would be to use a gibberish language like Simlish that is used in The Sims. I'm not sure how Simlish is implemented (they may have actually recorded whole sentences), but I imagine that with a reasonable number of word-like sound clips that can be joined together seamlessly in a random order, you could probably achieve a similar effect. Then you would just have variants that allow you to express making a question, anger, joy, surprise, etc. The subtitles would then be a "translation to English".