They also have a specification for maximum file size, so that limits your ability to post very long chapters at super-high MP3 bitrates.
Can you point to someone specifically that said ACX would accept WAV? I said recently that you should not do production in MP3 because the posted bitrate and the actual sound quality will not match. The show sound quality will be lower than you think. So your production and archive should be WAV, but the post will need to be MP3.
“Each uploaded file must be 192kbps or higher MP3, Constant Bit Rate (CBR).
Why? Before going on sale in the Audible store, titles are encoded in a variety of formats that customers have the option of downloading. 192kbps (or higher), Constant Bit Rate MP3 files are required so this encoding process works without error. You may upload 256kbps or 320kbps files if you’d like, but the difference in quality heard by listeners will be negligible.”
Well, the last statement contradicts my experience, especially if the file happens to be stereo and is reencoded, as ACX does.