Should I "call" teacher on unfair practice?

Your comment about "children of teachers" does sound fishy. -I would ask for some clarification in this dubious 'selection process'  

I would instruct my CHILD how to ask that question (and practice with her until she's comfortable), but would not interfere myself (because that would cause you to be viewed as 'one of THOSE parents' -- a label that will haunt your child for the rest of her school career).  

But it's not always about the best voice.  There may be some validity to your thought that the children of other teachers are being favored.  Or--and I'm just throwing this out there--it may be your daughter's attitude.   If she behaves like she is "all that" (and voices your opinion that she is a better singer than the girls being chosen), she may be passed over based on that alone.

But--in either case--I highly recommend that you do NOT intervene.  

We are often confronted with the challenges of being overlooked because someone who makes the call has another preference.  It happens in the workplace and unfortunately school.  I feel your dilemma as a parent, because my daughter who I know is an excellent drummer was overlooked and then dismissed when someone moved back to our church.  She is actually better than him.  So, did I get mad...yes.  But I redirected my frustration to focus on bettering her skills as a drummer.  No matter how great your daughter sings, her gift may not be well appreciated where she is.  You may have to look for other opportunities for her outside of her school.  Do you have local talent shows?  Can she perform at birthdays?  There are so many creative outlets, don't stop at the school.  No, it is not inappropriate to ask for the selection criteria.  If this is her passion, then no one can stop her.

I suggest not saying anything, even though you might not want to be "one of those parents" you are. It's possible that the teacher actually thinks the other kids are better than your child. If you don't think she is being treated fairly, why don't you sign her up with a children's choir outside of school where it will be more fair? We have to remember as parents that we see our children through rose colored glasses. It's great you care so much though, we can't say that for everyone.