I agree with your rule of thumb, but that seems like a positive description of the situation, not the normative one I'm looking for. You're giving me the conditions under which there will be outrage for discrimination (i.e., there exists a socially vocal/outspoken group that identifies with the quality being discriminated on). However, I was trying to tease out what the moral rule purported by such groups could be, since I see the "don't discriminate for how I was born" claim so often (and so forcefully put). I would suppose there should be some deep moral law being broken.
Would you advocate for the systematic mistreatment of group X if you didn't know whether or not you yourself would be born as part of that group?
For example, suppose I beleive in the systematic rejection of blind people from bus driving positions. I can see myself getting behind this even if I wouldn't know whether I'd be blind or not because I imagine behind the veil I'd put greater moral value associated with the safety of bus passengers than blind people's rights to the bus driving job. In other words, I've exposed the "driving" moral valuation behind my stance.
To go over your examples - the singing case is rather clear-cut, so is the math one (barring the Tiger Mother meme). Psychological damage starts entering into "neurodiversity" sphere. And about deaf people - there are many deaf communities which don't consider this as a disability, but as a different type of experience. In fact, the right for deaf parents to purposefully create a deaf child (when normally it would have working hearing) is a topic that's sometimes discussed.
It's not a very detailed explanation, but the rule of thumb is - when there's a vocal group of people who tie some quality X into their identities, talking about discriminating on X gets off-limits in the current culture.