Yes, the audio signal is still in the HDMI Out.
In my case, the audio signal (RCA Out) goes to the music system. To mute the projector (HDMI audio), I simply plugged a cable without a signal into the Audio In.

This is actually already stated in the product description. See also the product image.
3 options
- SPDIF via Optical Out
- SPDIF via RCA Out
- Analogue via stereo mini-jack out

Yes