D&D 5E bards and instruments

[77IM]

You don't need a special rule for Somatic. If a spell has both Somatic and Material components, the hand manipulating the Material components or focus can also perform the Somatic component. That's already a rule.

Regarding wind instruments: Nothing says that the Verbal component must be performed exactly simultaneously with the other components. Maybe you toot your horn and then say the magic word, or vice-versa. Furthermore, the section on Verbal talks about requiring "the particular combination of sounds, with specific pitch and resonance." So there's no reason the sound has to be produced by the mouth of a living creature. It's possible that when you cast using a flute, the sounds required are those produced by the flute. That interpretation is a bit more of a stretch, but it's consistent with the bard's story elements and not unbalanced, so I don't think it requires a formal house rule.

 

[Hriston]

Furthermore, the section on Verbal talks about requiring "the particular combination of sounds, with specific pitch and resonance." So there's no reason the sound has to be produced by the mouth of a living creature. It's possible that when you cast using a flute, the sounds required are those produced by the flute. That interpretation is a bit more of a stretch, but it's consistent with the bard's story elements and not unbalanced, so I don't think it requires a formal house rule.

This sounds good, but is not consistent with both the natural meaning of the word verbal and the description of the spell component which you have partially quoted.

Verbal (V)

Most spells require the chanting of mystic words. The words themselves aren’t the source of the spell’s power; rather, the particular combination of sounds, with specific pitch and resonance, sets the threads of magic in motion. Thus, a character who is gagged or in an area of silence, such as one created by the silence spell, can’t cast a spell with a verbal component.

The intent is clearly that words are required, not merely a set of pitches, etc. As a houserule, however, I think this reading has quite a bit of merit, and I would suggest that it could be extended to cover the verbal component of a spell cast with any instrument, not just those played with the mouth.

 

[77IM]

The part about "The words themselves aren’t the source of the spell’s power; rather, the particular combination of sounds, with specific pitch and resonance, sets the threads of magic in motion," seems pretty clear that it is the sounds, not the words, that are magical, which is why silence thwarts a verbal spell.

But like I said, that interpretation IS a bit more of a stretch, since you are right that the term "verbal" itself implies spoken, and therefore the most straightforward and obvious interpretation is that you have to speak the words.