Hi Julian,
I am as well using the Ultrastudio 4K Extreme... was looking for the same environment for audio monitoring and decided after a long search to integrate 3x BM Teranex Mini 12G SDI to Audio converters, feeding the analog XLR outputs into my 5.1 Genelec studio monitor system... not a cheap solution but works perfectly. After the latest firmware upgrade these Teranex Mini converters operate not completely quite but much better than before this update.
I can confirm that there is an issue with the audio transmission through HDMI. I am trying to feed my Denon home theater receiver via SDI out from the Ultrastudio and using a BM Teranex Mini 12G SDI to HDMI converter... video is perfect but channels 3 & 4 are swapped (Center & LFE) as mentioned before by others. I have tried to solve this severe issue through BM's support... they tried hard to find the mistake and offer a solution but for the time being I am still stuck with that... hopefully you find some solution, would be great to hear back!
Good luck
Hans