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.

We understand that. However, we are pleased to hear that you were able to find a suitable solution.

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>!