Line level detection using ADC from a passive summing box
simulate this circuit &ndash; Schematic created using CircuitLab

You will need to add a circuit like the one below on each input so that the ADC input will not go below ground.  There are protection diodes built into the part that will clip the lower half of the signal otherwise. 



The signal to the ADC will be centered at 1/2 of the V+ (2.5 V) and can swing +/-2.5 volts before the audio starts to get distorted.