singing vs solfege

singing and solfege both are nouns.

Word	Noun	Adjective	Verb	Adverb
singing	Yes	Yes	No	No
solfege	Yes	No	No	No

As nouns, solfege is a hyponym of singing; that is, solfege is a word with a more specific, narrower meaning than singing:

singing (noun)	solfege (noun)
the act of singing vocal music	a voice exercise; singing scales or runs to the same syllable
disclosing information or giving evidence about another	singing using solfa syllables to denote the notes of the scale of C major

singing (adjective)	solfege (adjective)
smooth and flowing