The rendering is not working because the tags used <> are rendered to special tags in Rich Text. There are a couple of ways to disable this functionality.

From inspector
Select the Value of Handle in each Slider’s Text Object from Project Hierarchy. In this case, there are four for each Slider: Music_Hovered, SoundEffect_Hovered, Ambient_Hovered, and Menu_Hovered.
Scroll down from the TMP menu from Inspector. Click on the Extra Settings to expand more options. There is a toggle for Rich Text. It removes the rendering of special tags on the slider by clicking on it.

From Code
In the script SptionsMenuSoundUi.cs where the Slider Texts are defined, disable the right text option by manually adding.
MusicLevelSelected.richText = false; to the TextMeshProUGUI text of each SliderUI.

Another way to do it, in the text of TMP, is to add the <noparse>
Tags in between so that the text is not rendered as special text.