IMO, voice alteration in general would undermine the concept, which is to hear genuine human voices. That seems like the opposite of what you want.

Have you considered limiting the number of voice messages non-paying users can post? As in, free users can post up to 5 messages per month, while paying users can post an unlimited number. Everybody can listen to as many messages as they want. The desire to "get your voice out" can be a powerful incentive to subscribe.