Mixer line in max voltage
It should be fine. If it's too loud, just turn down the gain in the mixer.

I don't know about the Behringer1 specifically, but most professional audio gear uses &plusmn;15&nbsp;V power rails on its line input and line output circuits, because peak signal levels can readily reach &plusmn;12&nbsp;V or more.

1 The manual states that it can handle +22&nbsp;dBu, which would be a signal that reaches &plusmn;13.8&nbsp;V peaks. (0 dBu is 0.775 Vrms, multiply this by 1022/20 and then by &radic;2 to convert from RMS to peak for a sine wave.)