Is There an Alternative to Resistor based Voltage Divider for an DC Audio Signal with Less Noise?
Lower resistance will give you less Johnson noise. I seriously doubt your 555 output is going to be affected noticeably by Johnson-Nyquist noise. It is 'white' noise which sounds like old time TV static. 
You may be able to avoid some wiper noise (when the pot is turned) by capacitively coupling to the divider since the 555 output is unipolar and thus effectively has an offset. That would be a scratchy noise when the pot is adjusted. 
Since you are drawing the signal from a digital circuit, any noise on the power supply or ground can cause noise in the output. If you are hearing hum that is mains-related (50Hz/60Hz/100Hz/120Hz) that is probably the source. 
Regarding your title-  you can substantially reduce the noise in a 'DC Audio Signal' by shorting the signal to ground, since we can't hear DC.