IPA: /ˈtjuːn.fəl.i/
KK: /ˈtjunfəli/
In a way that is pleasant to listen to, often referring to music or singing that has a nice melody.
The choir sang tunefully, filling the hall with beautiful music.
Tunefully → It is formed from "tune" (from Old French *ton*, meaning a musical sound or melody) and "-fully" (meaning in a manner full of). The word "tunefully" means in a manner full of melody or musicality.
Think of a 'tune' that is full of melody — that's what 'tunefully' means, doing something in a melodic way.