Dual led and transistor 3906 3904 switching with spdt switch on and off toggle help
A basic solution would be to just make a copy of this circuit and invert the driving signal which is being applied to the base of the second transistor. Here is a small schematic:

You could also spare one transistor and use just a standard diode instead. Here is an example:

Edit #1
Since the driving signal is an audio signal, the following circuit can be used to detect the presence of an audio. It basically consists of amplifying the audio signal and using a peak detector to store keep the output DET high if the audio signal is present. You could use this detector to drive one of the circuits above. Maybe a buffer withh be required between both circuits in order to not overload the detected signal.