SoX is the Swiss Army Knife of sound processing utilities. It can convert audio files to other popular audio file types and also apply sound effects and filters during the conversion.
I use also. "Better to go to 88.2 with SoX and 44.1 KHz input files. Less rounding error due to integer conversion and the internal word length used by SoX. At least that is my experience. No dither needed when adding more bits to the output file. YMMV, etc." It is from audiostream.com It is problem to do GUI for WIN?