To the only job of converting between different ratings I use "Free WMA MP3 Converter", a free program which works fine with my slow computer. Here is it, available to download-and-install: http://rapidshare.com/files/24936658/WMA_MP3_Free_Converter.rar
For editing (with filters, echos and etc.) I use Nero Wave Editor. Not very original, but it works
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