depam
Sorry for the slow response.
In this case using the standard repositories will probably not work. AFAIK enabling the debian repos you used in 3.4.3 will mess up 6.0 . Did you spot this link inside the link provided earlier?
http://skulboxx.com/Ubuntu/sbx/
After removing the existing ffmpeg and installing this deb with Kpackage ffmpeg was now able to do mp3. Don't know what other consequences installing this package may have.
While LQ (imho of course) is the best general linux forum, Mepis specific questions go largely ignored. Have you considered taking this question to a Mepis forum (hint: mepislovers)? The folks there may well know a better way to get this done.