If you're worried about differentiating voices, then WTNV is a great place to start with radio drama - like I said, it's not exactly normal drama, it's got a lot in common with narrated books as well. It plays like a community radio show usually, with the host giving news reports, ads, etc. For the first 15 eps there are no guest voices at all, only Cecil (giving accounts of what other people in town are saying.) After that there are guest voices maybe every 3-4 eps, but usually only one and in very limited roles (often in the form of someone calling into the show or somesuch). This ep had a huge cast but it's pretty much unique in that, and by the time you reach it you'll know who everyone is!