Would it be OK if the audio signal doesn't pass-through the Arduino? The Arduino doesn't have a proper DAC (digital-to-analog comverter) built-in, the ADC is only 10-bits, and there may be sample-rate limitations. (And, you cannot directly drive an 8-Ohm speaker without an amplifier.)
If you just want to switch the signals, you can detect the 2nd signal with an analog input (with the threshold software-adjustable), and then switch the signal path with a relay (use a DPDT relay for stereo).
Any tips or guides regarding negative voltages and outputting a whole signal to a speaker would be fantastic, but there are several great guides just dedicated to audio signals.
The standard approach is to bias the input at 5V (2 equal-value resistors and a capacitor), and then filter-out the bias from the output with a capacitor.
If you just need to detect the signal and it's OK to ignore the negative half-cycle, you can use a pair of [u]protection diodes[/u] and a resistor (2nd figure). For line-level signals, change the resistor to 10K or more. For a speaker-signal, 1K should be good. You'll also need a "pull down" resistor between the Arduino's analog input and ground. That resistor should be at least 10 times the series resistor value.