If you have thousands of files, then this will take forever.
find . -iname "*.wma" -execdir ffmpeg -i {} -ab 192k -map_metadata 0:s:0 {}.mp3 \;
(Older versions of ffmpeg may need -map_meta_data instead of -map_metadata, 0:0 instead of 0:s:0.)
(Newer versions of ffmpeg can use -map_metadata 0 -movflags use_metadata_tags instead of -map_metadata 0:s:0)
I tested this on Ubuntu 16.04. If you haven't already, you need to install the packages ffmpeg and libavcodec-extra-52.
Start this command from the parent directory that contains all your WMA files. It will search through all subdirectories for any file with a .wma extension and attempt to convert it to MP3. If the source file is named Awesome Song.wma, the new file will be Awesome Song.wma.mp3 and will be in the same directory as the source file.
If you normally user a bitrate higher or lower than 192k, change the -ab 192k flag to whatever you want.