I've only listened to the first couple of episodes myself, but I've heard The Silt Verses spoken of pretty highly by some TMA fans.

The Silt Verses is a full-cast audio drama that lurks in the grey area between horror and contemporary fantasy. Carpenter and Faulkner, worshippers of an outlawed god, are travelling up the length of their deity’s great black river, searching for holy revelations amongst the reeds and the wetlands. In these forgotten rural territories, new and strange gods of have flourished and Carpenter and Faulker must navigate this world of ritual, hidden language, and sacrifice. As they find themselves under threat from a police manhunt, the question remains: just how deep does their faith run?