The best tool to undertake the task at hand is the simpliest one that handles it
This freeware program helps one into easily batch convert audio files (.mp3, .wav, or .wave) with a multitude of options, like: the quality of the encoding process, the number of channels (mono or stereo) of the converted audio, the sample rate of the converted audio, the bits' depth of the samples of the converted audio, the type of extra info to add to the audio files, and (for me, the most important option) the normalization of the audio's volume of the converted audio (so that all the converted audio files share the same maximum volume, and there are no highs and lows when the track to be listened changes).

In addition, this program, as its name implies, can keep state, so that a batch job can be paused and, then, continued, at a later time.
Windows XP, Windows Vista, Windows 7, Windows 8, Windows 8.1, and Windows 10
Portable installation:
Uncompress the zip file to the folder from where you want to run the program (assuming you have both write and execute permissions on that folder) and run it from there.