Smallest audio amplifier?
You don't need to MUX audio.  Just mix the two audio sources together.  Maybe have the external audio muted. 

Why are you using a discrete-transistor buffer?  Use an op-amp for a buffer.  Better sound quality, fewer components, smaller overall footprint.

Here's a design that does what you want.  To amplify external audio, switch the micro pin to an analog input.  Switch it to a DAC output to amplify internal audio.


Input voltage 1Vdc+1Vp-p
Output voltage 1Vp-p
Load impedance 600 ohms


The value of C1 depends on the lower limit of frequency you care to amplify.