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To convert WAV to MP3 format (or to any other supported format) with FreeRIP WAV to MP3 Converter, you must use View->Converter menu option to set FreeRIP in "Converter" mode.
Once FreeRIP is in Converter mode you can use File->Add files to convert... menu option to build the list of Wav files to convert. If you prefer, you can use Windows drag&drop: drag audio files from a folder and drop them onto FreeRIP window.
Once the source file list is complete, use Rip menu to convert them in the chosen output format. E.g. select Rip->Convert selected tracks to MP3 to convert to MP3.
If you have multiple format audio files to convert to MP3 (or another format), you can add them all to FreeRIP WAV to MP3 Converter interface and bulk convert them.