Could hold person be cast as part of a speech?
My party is facing a more than deadly challenge due to some errors we made, and I want my character to give a small speech in which, some part of it will include a hold person cast on the most powerful enemies. Would that work by RAW?
The verbal component of a spell is described as:
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.
So you wouldn't be able to continue speaking normally, in a coherent fashion. Although you would continue making sounds it would appear obviously disjointed to the rest of speech spoken in whatever language you were using.
You would also be hampered by the somatic and material components of the spell so if your aim to disguise the casting of the spell, it would be very difficult to hide your intention from another caster.
As @Steve pointed out in his answer, spells with verbal components would most likely not work like that. You could try to get the DM to allow a deception check to try to get the creature you are fighting to believe that is how you speak.
If you are a Sorcerer you could use metamagic to cast Hold Person using Subtle Spell. You wouldn't need to speak at all but you would be allowed to if you chose.
Subtle Spell (PHB pg. 102)
When you cast a spell, you can spend 1 sorcery point to cast it without any somatic or verbal components.
From Out of the Abyss there is a magic Item that could do what you want
Spell Gem (OotA pg. 223)
...While holding the gem, you can cast the spell from it as an action if you know the spell and the spell is on your class's spell list. Doing so does not require any components and doesn't require attunement. The spell then disappears from the gem...
If you just want to cast a spell to stop the creature, the College of Glamour Bard has a feature called Enthralling Performance that grants the ability to cast an altered version of Charm Person. Granted, you could not do this in one round.
Enthralling Performance (XGtE pg. 14)
Starting at 3rd level, you can charge your performance with seductive, fey magic. If you perform for at least 1 minute, you can attempt to inspire wonder in your audience by singing, reciting a poem, or dancing. At the end of the performance, choose a number of humanoids within 60 feet of you who watched and listened to all of it, up to a number equal to your Charisma modifier (minimum of one). Each target must succeed on a Wisdom saving throw against your spell save DC or be charmed by you. While charmed in this way, the target idolizes you, it speaks glowingly of you to anyone who speaks to it, and it hinders anyone who opposes you, avoiding violence unless it was already inclined to fight on your behalf. This effect ends on a target after 1 hour, if it takes any damage, if you attack it, or if it witnesses you attacking or damaging any of its allies. If a target succeeds on its saving throw, the target has no hint that you tried to charm it. Once you use this feature, you can’t use it again until you finish a short or long rest.
Hold Person has the full triad of components; Verbal, Somatic, and Material. As defined by the general rules for casting a spell, these components are fairly obvious:
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.
Somatic (S)
Spellcasting gestures might include a forceful gesticulation or an intricate set of gestures. If a spell requires a somatic component, the caster must have free use of at least one hand to perform these gestures.
Material (M)
Casting some spells requires particular objects, specified in parentheses in the component entry. A character can use a component pouch or a spellcasting focus (found in “Equipment”) in place of the components specified for a spell. But if a cost is indicated for a component, a character must have that specific component before he or she can cast the spell.
At the point at which you want to cast hold person, you will have to start chanting mystic words, making some sort of forceful or intricate gestures with at least one hand, and you will have to pull out and manipulate a small piece of iron or else a spellcasting focus like a wand or other tool appropriate to your class, and it will be pretty obvious when you do so. Even for the verbal component, "mystic words" is obviously not meant to mean normal speech (though some GMs/tables occasionally interpret spells with verbal components that way when it is thematically appropriate, such as for the Suggestion).
It's not possible for you to somehow surreptitiously cast a spell like hold person as part of a speech, unless you have some other ability which allows you to cast hold person without components or obviates your need for spell components generally. For example, a 20th level druid's archdruid class feature states that:
... you can ignore the verbal and somatic components of your druid spells, as well as any material components that lack a cost and aren’t consumed by a spell.
Such a character could cast a spell without any outward signs at all, and nothing seems to preclude them holding a conversation while they do it (I would assume any 20th level caster has had plenty of practice concentrating while multi-tasking).
The other answers are correct on the most basic element of this question: the verbal component of the spell isn't plausibly a syntactically valid construction in Common (or any other language in the D&D settings, most likely).
The way that you can try to accomplish this is with a speech that provides cover for casting the spell. For a shallow example, imagine someone giving a speech in English while gesturing emphatically and then reciting a verse in Latin. The structure of the speech would have to make that Latin verse recitation seem natural to avoid drawing suspicion, but it's the kind of thing that a lawyer might be able to do fairly naturally.
If your character can present some reason or provide some structure to their covering speech, they may be more or less likely to get away with the deception. The typical way that something like this would be handled is with a Skill check, likely Deception or Performance in this case, with your DM setting the difficulty to get away with casting the spell before the audience realizes what you're up to.
Depending on how your table handles PC-vs.-Player plans, the DC may be modifiable if you describe a cunning strategy or your character makes an Intelligence or Charisma check with good results.
Some class-specific options include class features like the Sorcerer's Subtle Spell metamagic, which would make this far easier to pull off, but as a general guideline you should not expect to get the benefits of a class feature you haven't taken through a workaround, however clever. Consequently, even if your DM allows this, don't expect it to be a frequent option (unless you take enough levels in Sorcerer to gain Subtle Spell, for example).
