What does this transistor, parallel to the audio output load, do?
It's a muting circuit.

When the +5V is present it turns the transistor on and shorts any audio from the preceding circuit to ground.

The 5V signal will be set to ground or negative to allow audio to pass.

The audio must not be more than a few hundred millivolts peak to peak or the transistor may turn on during the peaks of the music and cause distortion.

It may be used to ensure that no audio is output when the other circuitry is powering up, being initialized or the configuration is being changed in some way.

It is not commonly appreciated that BJTs can operate with the reverse polarity on the collector. The gain is usually very low but is useful in circuits such as this.