Previously, I had mentioned the audio versions of Seneca's letters that Tim Ferriss worked on with John Robinson to put together, which are linked to below. There was an audio quality issue with the "On the Shortness of Life" essay that was included at the beginning of Volume 1 which received some criticism (deservedly so). I'm not sure if it's fixed yet, but I don't believe the issue was widespread throughout the rest of the recordings. So what I'd recommend if you are interested in getting the letters is to listen to the sample of the version of the letters above, and then listen a sample of the ones linked to below, and see which narration your own ears prefer.
If you want to top off your library with the other well-known work of Stoicism, here is the narration I've liked best so far of the couple I've listened to: Meditations - by Marcus Aurelius.
And if you want an overview of Stoicism before getting into the works of the key figures, William Irvine's A Guide to the Good Life: The Ancient Art of Stoic Joy is a good intro.