To be able to use the internal sound of the Rabosapien as well as audio from the PC on the Robosapien's speaker, an analog switch is needed.
The 74HC4053 will be used. It's cheap and easily available. Its three internal channels are connected in parallel. In this application it doesn't matter much, but it decreases the series resistance of the multiplexer.
The audio from the laptop is too weak to connect directly to the Robosapien's speaker, so we'll use an audio amplifier. The PAM8403 is available on a PCB with the necessary peripheral circuitry. These modules only cost a few cents. The audio amplifier is connected at the output of the multiplexer.
The audio output of the Robosapien can be taken from C13A. This output is connected to a 10/2K2 voltage divider, which is connected to the analog multiplexer. Decreasing the Robosapien to 1/6th of the original level is needed to avoid oversteering the amplifier.
Finally the Digistump controls the state of the analog multiplexer, choosing between PC audio and Robosapien audio for the Robosapien speaker.