How do I create a custom voice, and can its tone be changed?

Voices cannot be generated in vanilla Minecraft. They are custom sounds added by mods.

All the commands you tried are for printing messages in the chat. /playsound is the only command that can play sounds, and it can only play predefined sounds. You will need to add your own voice as a custom sound using add-ons.

I'm not much of an expert in modding Minecraft, so I'm not the right person to ask for the job. Check out info on add-ons if you want to explore this path further, or you can wait for the next answer from a more experienced Minecraft modder.

Basically, first I recomend going to minecraft.net/en-us/add-ons and downloading the “vanilla resource pack”. Then you can do two things. One thing you can do is change a vanilla sound to an OGG file. You cha find a tutorial on this online. Or you can actually create entirely new sounds with just resource packs and then use /playsound to play it. I’m not gonna explain the new sound one because it takes multiple JSON files to write and you might? be able to find a tutorial but you might not. I recomend the first approach that should be pretty easy.