Here are various methods you can pick from to convert audio to MP3:
Drag the file from File Explorer to the Reaper timeline. Go to File > Render. Type in a new filename if desired. Click “Browse”, choose “Browse for directory”, choose where to save, then click OK. At the “Output format” drop-down, select “MP3 (encoder by LAME project)”. Click “Render 1 file”. When done, Reaper reports “Finished in…” within the “Render status” area.
In Adobe Media Encoder (video below)
Drag the file from File Explorer (or from Finder, if on a Mac) to the queue area of Media Encoder. Under the format column, collapse the drop-down menu (triangle on the left, under the filename) and select “MP3”. Under the “Output file” column, click on the blue text to choose a location for your converted file (and type a new filename name if you like), then click “Save”. Press the green play icon in the top right of the queue area. When finished, Media Encoder reports “Done” in the status column.
In VLC Media Player
Go to Media > Convert / Save. Click “Add”, navigate to the file, then click “Open”. Click “Convert / Save”. In the window which appears, collapse the dropdown menu near the word “Profile” and choose “Audio – MP3”. Click “Browse”, navigate to where to save the file, then type a name in the “file name” bar. Click “Save”, ensuring that the “Save as type” drop-down menu is set to “Containers (*.mp3)”. Click “Start”.
Open the file in iTunes. Go to iTunes > Preferences > General. Click “Import Settings”. For the “Import Using” dropdown, choose “MP3 Encoder”, and click OK. The next step depends on your version of iTunes: Either right-click the file within iTunes and choose “Create MP3 version”, or select the file within iTunes and go to File > Convert > Create MP3 version. When iTunes has done this, a new file appears: Right-click this file, and choose “Reveal in Finder” to show the location of the file.