Why does being deaf give you a spell failure chance?

[Yqatuba]

Even if you can't hear yourself talk, you can still make the mouth movements, right?

[Quixote]

Right.
...most of the time. But about 1 in 5 times on average, your newfound lack of hearing causes you to make some small error in what I assume is an extremely delicate process.

[Sandslice]

Yqatuba wrote:

Even if you can't hear yourself talk, you can still make the mouth movements, right?

I'm not sure what your native language is, but you might try speaking Vietnamese without being able to hear what you're doing.

There's also the problem of minimal pairs. If you say "Klaatu varata nichto" instead of "Klaatu barada nikto" because you forgot to curb your native accent while casting, you might turn your familiar into a newt when you meant to cast fireball.

That's why the Deafened condition causes intermittent failure in verbal-component spells.

[Belafon]

Assuming you are not a deaf person, have you ever had something happen that temporarily impaired your hearing? Loud bang, water in the ear, or the like? One of the side effects is that this alters how you hear your own voice, and you often overcompensate by speaking too loudly or changing your vocal pitch. Even moreso if you can't hear yourself at all.

Since magic requires precise hand movements and precise spoken incantations, it's not unreasonable to have a spell failure check while deafened.

Especially if you are not used to being deaf. As the Deafened condition states: "Characters who remain deafened for a long time grow accustomed to these drawbacks and can overcome some of them." That's deliberately nebulous but most GMs will either gradually reduce the failure chance or create a feat that mitigates the chance for deaf characters.

[Yqatuba]

I actually haven't ever been deafened so I don't know. I do think the idea of spell words working like a tonal lanaguage might make sense though.

[Scavion]

Something my wife has caught on to me for is that my noise cancelling headset makes me talk way louder on voice chat than I probably would in normal conversation.

Volume, pitch, intonation and not to mention accent. Magic requires a level of precision not easy to maintain when completely deafened. Considering you can't cast stealthily, intentionally trying to vocalize quieter would probably fail the spell too. The Silence spell also completely shuts out vocal components so it's pretty clear that it needs to be audible.

[Kimera757]

Yqatuba wrote:

Even if you can't hear yourself talk, you can still make the mouth movements, right?

I knew a woman with a hearing impediment at university. She could speak in English and at least one other language (plus sign language), but her English sounded strange. It was not an accent. She just wasn't able to speak it properly. She could not hear what she was saying and "auto correct". Fortunately I could understand her most of the time.

I do not know how common this is among the hearing-impaired.

[Mudfoot]

I always noticed that I seemed to speak with a lisp when my ears were still ringing after a loud rock concert. I'm not sure if I was really lisping or it just sounded like that.

There might be a more metaphysical explanation: you can't sense the magical forces gathering if you can't hear and feel them. But as blindness doesn't cause that effect, possibly not.

[Cavall]

I've always thought of it as a massive ringing in your ears that cause strain and distractions. Like sonic blasting your ear drums.

[marcryser]

The default assumption to the game is that characters can hear (and see, smell, etc.) It is suddenly being without that hearing that is disorienting and confusing and, therefore, gives a penalty.

A character who is deaf and has been that way would find it more difficult to learn verbal magic, especially from a mentor who can hear, but would be no more likely to mess up their spells during casting than a normal caster would. They have LEARNED to cast without hearing.

There is no rule support for that because of the initial design assumption.

[TwilightKnight]

Mudfoot wrote:

But as blindness doesn't cause that effect, possibly not.

Most spells require you to designate a target. Being blind prevents that. So there is no need to add a failure chance because for most non-self spells, the failure will be 100%.