To fix MP3 playback issues on your Raspberry Pi 4B with mplayer, try using the plug device for format conversion:
bash
mplayer -ao alsa:device=plug=plughw=2,0 /home/ianglei/music/horse.mp3
Make sure libmpg123-0 is installed, check your ALSA configuration, or use -ao pulse as an alternative. Like troubleshooting Spotify Premium playback, debugging with aplay can help identify supported audio formats.