Divine casters are not affected by deafness, it is only bards/sorcerors/wizards who receive the spell casting failure.
Deafness also means they won't be affected by bardsongs, or be able to detect sneakers through a listen check.
I have a hunch that Bioware implemented the spell failure as your regular armour induced spell failure. That's why clerics and druids are not affected by it.
So, without having tried it, if you'd use still spell you should be able to negate the effect.