I was able to convert mp3 files to wma.
First you need to have Wine(http://www.winehq.org/) installed which lets you install and run a host of windows application in Ubuntu. I'm using Ubuntu 804.
Use Wine to install and run JetAudio.Plus v7.1.0.3100.(http://www.cowonamerica.com/products/jetaudio/)Maybe another version of Jet Audio will also work(I don't know).
Once in Jet Audio open the playlist and go to 'add files\add files in folder' and load the folder with the mp3 files you want to convert.
Once you have all the files in the playlist you 'select all'(Ctrl+a)\right click on the files and select 'Export', type in a file name and save. Close the playlist window. Now open the Convert Audio in Jet Audio and Import the .pls file you created of the files you want to convert. Note that this was the only way I found to import many files at once into the convert audio window. For 'Output format' chose WMA Windows Media Audio 7 (9 didn't work for me), configure your bitrate and your good to go.
Have fun.
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