After looking around a simple and efficient solution is probably to use FFmpeg Batch (and it’s free).
FFmpeg Batch is a GUI for FFmpeg.
There are 32 and 64 bits packages, portable or not.
Do allow encoding with multiple output file formats at the same time, and can process multiple input files at the same time to better use multi core processors.
Use something like this for wav to mulaw :
ffmpeg -i input.wav -af "highpass=f=300, lowpass=f=3400" -ar 8000 -ac 1 -ab 64k -f mulaw output.ulaw
The only missing feature is watch folder (hot folder).
If you need this feature then it’s better to use a watch folder utility (Robotask, Watch 4 Folder…), in tandem with FFmpeg command line.