You don't have problem with this one. In the hungarian language we have a very flexible word order (with negation too, but you need to be connected to the other part of the sentence). The order depends on what do you want to highlight.
The matter is WHAT you are negating. In the current example you negate itt (nem itt, hanem ott). In your example you negate énekeltek (nem énekeltek, hanem hegedültok). Nem must stand right before the negated word.