Singing in the harp
The soundboard of the harp can be used as a resonator for sounds produced with the voice. The harpist sings or talks into the sound holes at the back of the harp's soundboard.
Singing into the harp.
The soundboard is very responsive to exterior sound sources. For example, the orchestra harpist must repeatedly damp the resonance of the harp after loud passages in the orchestra, even when he/she has not been playing himself/herself. The soundboard and the strings start to resonate and can sustain the resonance for quite a long time.
A verbal explanation is necessary.
The back of the soundboard has several sound holes. It does not matter into which of them the vocal sounds are produced.