Hello Neil, are you able to play the original .wma files using your media player, or on Switch before you convert them? If you are not able to play those files, then converting them won't fix the issue.
Also, when you are converting the files to mp3, click on Options next to the output format option (mp3), use the constant bitrate, set it to 192, and click on OK. Convert the files and test to confirm if they play fine on the computer and mp3 player.