With one group of friends I’ve been doing weekly calls for months now with audio through Mumble for audio, and video through Discord where everyone stays muted.

Are you doing instructions, including "your hand position isn't right" etc.? For that latency is tolerable, but different alten y for picture and audio is problematic.

Sometimes, but only for drinking games and such.

As a non-musician, could you explain why?

How you place your hands, bend your wrists, stretch your fingers and generally hold your instrument can greatly impact how you play it. Depending on the instrument, a simple bend of a wrist can lead you to consistently play out of tune, or make it hard for you to hit a specific note.

Even just touching strings in different ways can produce wildly different results; think playing a harmonic note on a bass guitar, which is how you get such a low-sounding instrument to play a high-pitched note. How you blow air through your horn can greatly change the timbre and mood of your music.

There are a lot more examples, and they are all difficult to correct if you aren't able to see what the musican is doing.

It's similarly essential for teaching voice.

The exception here is at the MasterClass level. Fuck the world for autocorrecting that to camelcase.

Physical matters of technique. Depending on the instrument or lack thereof: grip, placement of fingers, posture, breathing, lip embouchure, and more. Heck, you could even have it put together wrong or be holding it upside down. A remote teacher cannot easily convey this things in a timely manner without seeing the student.

> Music instruction cannot possibly be effective without a visual component

I suppose blind people will never be successful with music.

Please re-read my post and note that, at the beginning of the sentence you quoted, I specifically targeted the instruction of music. More specifically, you cannot help teach a blind violinist how to prevent themselves from continuously playing their notes flat without being able to see how they are playing.

Blind people are perfectly capable of learning to play music. Blind people and non-blind people alike can learn on their own, or seek out instruction. It is the latter that we are discussing here.

Could a blind person be a music teacher?

Totally, and I'm sure there have been many, but being able to touch and feel the placement of the musician's hands/fingers/etc. would be important. There are certainly things they could teach without that touch, just like non-blind people could teach a few things remotely sans video, but core areas of playing wouldn't get the right attention.

I think they were referring to remote music instruction....