thats an exception, because in the games P's personality is based off you and the choices you make, basically you are the producer himself.
now in the drama cd, you cant make choices or alter the course of the story and many people are hearing the same thing so they adjust the personality to everyones needs and gave him a voice.
P did not have a voice in eternal prism series because the only time he ever had a presence was in eternal prism 3.
......Why can't she, or the rest of the cast for that matter, have voices for Drama CDs? Think of it this way: In said DCD, the producer is a character that interacts with the rest of the cast through verbal communication. Since it's an audio media, and communication made through visual interaction can't be expressed, the use of sound is required to prove he 'exist' and is part of the cast.
That applied to anyone who may get an appearance in Drama CDs- they will be voiced regardless. No exceptions to whoever will be. If I recall correctly, on one Drama CD, there was a Drama CD-only character intended for plot purposes. If they can go that far, why shouldn't in-game cast members be? >.>
(We're not talking about DS anymore, huh?)
On-topic: What's the worst that happened on a RyoXYumeko relationship (in-game)?