Mixing it up

I bought a mixer the other day, to DJ records with. But it went a bit wrong, so I had to take it back to the shop.

"What seems to be the problem?" asked the man in the shop.

"Well, when I mix between records, I can hear someone shouting at me angrily. And when I try and adjust the balance control, someone calls me a tosser."

"Ah," said the man, "That'll be the cross fader."

"And the insults?"

"Well, the balance control's definitely a knob."