The Optoma UHD38x projector does not have Bluetooth for sound. Instead, it offers a USB-A port to power an HDMI dongle such as Google Chromecast or similar devices, and there is also an integrated speaker for easy audio output without the need for external speakers.
I realised that the built-in speaker is not really great. And since it's a front projector, you always have the problem that the projector speakers are behind the guests and the image is much further in front. I have now ordered a 20m jack 3.5mm cable for my external system. It's always daft when the projector has to be at the back and the speakers at the front. That would work with a dongle, and with the extension cable too.
In the meantime, all projectors should actually be equipped with Bluetooth, as everything is becoming more and more decentralised. But all good>!