At the moment I'm using an amplifier based around a NPN transistor and some resistors. Its configuration is as an emitter follower. Pin 25 supplies internal audio from a DAC on the chip.
This handles DC offset (offsetting the AC signal into a DC signal), overvoltage input protection, switching between internal/external audio, and output. It is a bit distorted, but it works well.
Ideally, I'd like to integrate this on to one or two IC's with the minimum space usage. It'd be nice if cost was lower, but this is not critical. (Currently component cost is around $2.50 total for all components in the audio amp, in low quantities.) Also, it would be ideal if the IC did not require a dual rail power supply.