Does the projector have a volume control?
Some projectors will only output sound from the analog audio inputs not the HDMI input. Pretty stupid but it saves them some money. You can check this by connecting your phone to an audio input on the projector.
If this is the case your options are
1. Buy an HDMI input selector with audio extraction to connect the speakers.
2. Convert the coax digital output of the bluray to analog with a digital to analog converter.