audio blend circuit / emitter follower
For selecting proper value of resistors you should have at least two option : Vce_saturation and Vbe_on.  Lets assume your VCC=5 volt , Vbe_on = 0.7 volt and Vce_sat=0.2 volt.

For Maximum output swing we should set Ve voltage to :

Ve = (VCC - Vce_sat) / 2 
Ve = (5-0.2) / 2 = 2.4 volt.

So Vb = Ve + 0.7volt = 3.1 Volt.

Then we need to set Rb1 and Rb2 to achieve this voltage. 

Vb = ( Rb2 / (Rb2+ Rb1) ) * VCC

Rb1/Rb2 = VCC/Vb - 1 = 0.61

Using Normal values for resistors : Rb1 = 33K and Rb2 = 56K .

assuming Ie = 1 mA : Re = Ve / Ie = 2.4K . that Using a 2.2K resistor is sufficient.

And as you now Voltage gain of CC configuration is about unity.