$ ls *mp3 | xargs mplayer Playing Lemon. File not found: \'Lemon\' Playing Tree.mp3. File not found: \'Tree.mp3\' Exiting... (End of file)
xargs on MacOS doesn't have -d option, so this solution uses -0 instead.
Get ls to output one file per line, then translate newlines into nulls and tell xargs to use nulls as the delimiter:
ls -1 *mp3 | tr "\n" "\0" | xargs -0 mplayer