"Priority", as in the presence of one signal switches off another, is a bit difficult to put together.
Why not just mix them all together? If one of them produces no sound, it won't affect the others. Best way to do this is an inverting amp, TL082 will do the job nicely. Just apply any number of inputs, with a resistor in series with each source, to the inverting input. The resistor will set the volume for that source and a feedback resistor from the output back to the inverting terminal will set the overall gain. You will probably need a capacitor in series with the sources and output too, if you want to avoid needing a negative voltage supply (this is desirable).