Here's why:
* Concessive clauses express a contrast or concession. They acknowledge something that could be considered an argument against the main statement but then go on to state the main statement despite that concession.
* In this case, the main statement is "we have no soprano positions left". The concessive clause "although you sing well" acknowledges the speaker's talent, but ultimately, the lack of positions is the deciding factor.
Let me know if you have any other clauses you'd like to identify!