I have now had a chance to test this new functionality. Process required below
1. After upgrading to latest firmware, On B2 change compression mode to FLAC+MP3 (not available on webui yet)
2. Ensure NO browsers anywhere connected to B2
3. Select Compress now from Settings/Compress now, you will see red light flashing away for a VERY long time.
4. Overnight my B2 has only managed to compress the first 22 albums, it will stop when you use it, when it goes back to standby, process continues.
5. to grab your MP3 files use Playlist/Export MP3, select playlist you want and all MP3 tracks there will be sent to the USB stick connected to USB C the format is Artist/Album/Tracks so should work just fine in your car etc.