This can happen on many TV’s when using an HDR-2000T without one of the later software versions.

Updates no longer are transmitted via your aerial which the “update” in the menu used to rely on.

You could try connecting your HDR-2000T to the internet via an Ethernet cable and a router. For most it takes less than 30 minutes for the HDR-2000T to realise there is more recent software but some have reported it not noticing until the next day. But if you have already done this and declined the offer to update then it doesn’t appear to offer an update again.

The remaining option is to download the software from Humax’s support site and transfer to your HDR-2000T via an usb memory stick.

https://uk.humaxdigital.com/support/?product_id=794&curr_page=sw_release

To check which version you have its MENU -> Settings -> System -> System information.

It should show Software Version UKTFAC 1.01.20

For extra discussions on your topic there are plenty of other threads already started.

They are easy enough to find but here are a few for starters if you want to see what others have said. On all bar one, the issue was fixed after updating to a more recent version of the software.

https://myhumax.org/forum/topic/picture-but-no-sound-on-humax-hdr-2000t

https://myhumax.org/forum/topic/no-sound-10

https://myhumax.org/forum/topic/tv-sound-loss-when-recording-2-channels

https://myhumax.org/forum/topic/humax-1800t-sound-issue-update-download-problem

https://myhumax.org/forum/topic/hdr-2000t-sound-problem

https://myhumax.org/forum/topic/no-sound-3/page/13

https://myhumax.org/forum/topic/sound-problems

https://myhumax.org/forum/topic/loss-of-sound-1

https://myhumax.org/forum/topic/no-sound-humax-hdr-1800t/page/4

https://myhumax.org/forum/topic/no-sound-when-turned-on/page/2

https://myhumax.org/forum/topic/h2000t-sound-dropping