This shouldn’t affect audio, but you need to set the GUI resolution to 1080 instead of 4K. As to the audio problem, please post full debug logs.
We recommend people with 4K TV’s set their user interface (UI) to 1080p. Kodi’s UI is not optimized for 4K yet and this can put unnecessary demands on your device and can lead to a suboptimal picture quality, as well as potentially cause other issues.
The settings we recommend are as follows…
Settings>Player>Videos>Adjust display refresh rate> On start/stop
Some televisions may also need, or benefit from, the following being set…
With the above settings your UI will be output in Full HD and your 4K content will be output in 4K. If you have any doubt, feel free to upload some logs so we can verify that your settings are indeed correct.