Hi, I'm not familiar with Rotter, just so as you know... For patent-restriction reasons Ubuntu doesn't ship with the MP3 codec - there are instructions on how to install it here https://help.ubuntu.com/community/RestrictedFormats I don't think the version of LAME in the repository contains the codec. See if that clears it up. You might want to try rendering in ogg vorbis to rule it out.
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