Intent matters here
. . . in other words, it's really a question of how the DM (and the players) want to roll with it.
After all, we don't really know exactly how magic item command words work.
First of all, it's magic, and second of all, we don't have a detailed specification of how command words trigger magic items. Is it the sound? Is it the intent? Some combination?
We know there are two steps:
- You learn how to use the magic item
- You speak a word or phrase
The basic rules say, under "Using a Magic Item":
A magic item’s description explains how the item works. Handling a magic item is enough to give a character a sense that something is extraordinary about the item. The identify spell is the fastest way to reveal an item’s properties. Alternatively, a character can focus on one magic item during a short rest, while being in physical contact with the item. At the end of the rest, the character learns the item’s properties, as well as how to use them. Potions are an exception; a little taste is enough to tell the taster what the potion does.
A kenku can clearly complete this step. Can a raven? The rules don't say. And if you teach a raven to make a certain sound, is that enough? After all, there's nothing in the rules that suggests you can trigger someone else's magic item just by saying a word, so it isn't just the sound.
And if it has a command word:
A command word is a word or phrase that must be spoken for an item to work. A magic item that requires a command word can't be activated in an area where sound is prevented, as in the area of the silence spell.
But this is very broad. You don't have to speak a specific language. Or don't even have to be humanoid. Surely a mind flayer or a lich or a ghost or a fiend speaking a command word all sound very different than a human. There are no rules in which a magic item says, "I'm sorry, I'm just not clear on what you're saying. Are you trying to say my command word? Your accent is just really very thick."
Kenku
It matters more whether the DM wants to saddle a race with an inability to use many magic items than the precise wording of the race.
And note that the kenku in Volo's is legacy. The current definition of the race says under language:
Your character can speak, read, and write Common and one other language that you and your DM agree is appropriate for the character.
So for the current race, it isn't even a question. For legacy kenku, what does the DM want?
As a DM, I'm not going to saddle a player with a character that can't use a broad swath of magic items, unless that's specifically what the player wants. Even then, I have to ask, is it really more fun this way?
As aherocalledFrog said in the comments, this seems like a chance to have fun doing a standard thing, rather than a reason to not allow a standard thing.
Ravens
Again, up to the DM. Does the DM want ravens to trigger magic items?
The rules just don't say. I don't think there's a player-exploitable loophole here . . . "I've taught my raven to trigger the Sphere of Doom and you can't stop me, because RAW!" . . . .
Broad strokes, even if the raven can imitate a word, is it really "speaking"? And even then, can it go through the necessary (and not well-defined) process of learning to use a magic item?
As a DM, I would not have a broad interpretation that ravens can trigger magic items as a matter of course. I might be willing to discuss it on a case-by-case basis . . . a specific character, a specific raven, a specific magic item. Maybe.