The notes that a musician stresses and emphasises in a melody, the contouring they use, their phrasing of the melody -- essentially, the way they articulate the melody: I call that "voicing". I'm using that in the literal sense ("giving voice to"), not the musical sense ("chord voicing").
Is there an actual music term for this?
'Phrasing' in music refers to the way the musician expresses emotions or ideas through their performance. This can involve dynamic changes, articulations, tempo changes/adjustments and use of performance techniques.
I quite like this page in explaining how to identify the elements that might influence the phrasing that should be used, and how to communicate through phrasing as a performer to the audience.
I think you've answered your own question by mentioning 'phrasing'. Yes, that's what it's called.
Don't call it 'voicing'. That has another musical meaning already.
Stressed notes would be tantamount to accentuation which is also emphatic. Tone colour, dynamic contour, and articulation are all part of phrasing. Other emphatic directions include the fermata (often used on a half cadence or cadence in a modulation). I suppose that tempo directions like rit. and acc. shape the temporal countour of any phrase. Interpretation is when an artist realizes* their incorporation of all these aspects in a performance. Thus, interpretation is really the only term that encompasses all of the components in your question.
*realizes - to execute; to make/create; or to bring to life.
