Discord can be integrated into the major game engines out there. I think Pantheon is using Unity (forgive me if I am wrong) and there is already a plug in for that (although I am no expert in Discord or its' dlls).
From my point of view, it would be better to have voice in game rather than having to go externally to do so. As for concerns about listening to others doing stuff you would rather not have to listen to, then, yes there should always be mute options.
I grew up on EQ1 and typing was essential and I touch type, however, I still feel voice is a far superior form of communication. There are points in a game where you should be concentrating on game play rather than spelling and grammer and trying to type to someone and keeping interrupts going or heals landing is sometimes distracting at best: "Chenz, move forward, hurry! Wanderer coming up behind you..." oh too late.
Text also doesnt easily provide context. The phrase, "Get lost" in text is a bit mmm.. one dimensional. In voice you can make is sound fun, or a question or an instruction, just by changing the pitch and duration. In text you need to change what you type "Get lost, lol", "Get lost?", "GET LOST!". Extra typing to get the same desired effect. Small in this example, but a sentance can be easily misread and I remember lots of situations where the true intention of the text was completly misunderstood.
Text is useful and important, but so is speech. I agree, I dont want speech all the time, but for me, but is should be integrated and it must be easy to switch on and off and I would like it integrated with a few tools at the outset. The minimum should allow switch between channels for world, area, local, guild, group, individual and be integrated with any player ignore functionality.
Raiding and grouping should allow for specific individuals to be the ones to direct to avoid confusion (and mute the rest - maybe with a toggle to switch between fight mode and rest mode), but I have been in many raids with voice on and they were successful without this kind of functionality.
Yes, it is development, but it is not horrendous and Discord provides the chat engine anyway. I think it would be worth the effort and add to the grouping and general gaming experience.