QWinFF is one of the best free batch audio conversion tools for Windows. Its interface is quite intuitive that makes it easy to use. It supports various audio as well as video formats (discussed inside).
After launching it, you can easily browse files to convert them to desired format. To do so, click on + button or go through File(menu) > Add Files (option). Now, it lets you set desired output format (AC3, AIFF, ASF, AU, AVI, DV, FLAC, FLV, MOV, MP2, MP3, MP4, OGG, RM, SWF, WAV, WMA, etc.). After specifying output format, it lets you customize corresponding settings to get desired quality, sample rate, bit rate, channels, volume level, etc. in converted audio files. At the same time, you get option to specify output folder as well. Now, as you click on Start button, it begins batch audio conversion process.