Foobar2000 is not just a good audio player, it can be used to convert audio files to any other file format, like MP3. To convert to MP3, Foobar2000 needs the LAME MP3 Encoder.
LAME ships only as source code. Compiling the source code on Windows can be quite a chore. Thankfully, pre-built LAME bundles can be downloaded from RareWares. The bundle ships as a ZIP file that includes the LAME executable (
lame.exe) and library (
lame_enc.dll). Foobar2000 uses the LAME executable for MP3 conversion. Unzip the LAME bundle in a suitable location.
Conversion to MP3
Open Foobar2000 and load the audio files that need to be converted to MP3. Select the files to convert in the playlist, right-click and choose Convert → Convert. In the Convert dialog, click on Output Format and choose MP3 (LAME). To set the MP3 bitrate, click Edit. Click Destination to set the filename format and the destination directory. Click Convert to begin the conversion. The first time you do this, Foobar2000 pops up a File dialog to allow you to choose the location where the LAME executable is located.
After this, the files are converted to MP3 and written to the destination directory. The MP3 conversion should be pretty fast since Foobar2000 spawns off one LAME encoder instance per processor core of your computer.
Tried with: LAME 3.98.4 and Foobar2000 1.1.1