I used to use musicbee for this. Good app for organising your music on your desktop and you can configure it to convert audio format when syncing to your phone. It’s been a while since I used it, but it did the job nicely

If you’re using an app to view lemmy, it’s worth updating regularly (at the moment anyway). Mine hadn’t automatically updated for a while, but it turns out Jerboa (on android) has made a lot of improvements to the app in the past few weeks