Mirror Screen to Smart TV can be used for various purposes - from watching films through browsing photos to showing presentations. Depending on the purpose, you will cast the screen (i.e. video) only or will also neeed to have the sound output from Mac audio. If you want to cast both, i.e. audio and video together, e.g. for playing movies, you will need to install dedicated audio driver.

The audio driver is a kind of virtual loop that acts as a standard Mac audio output. The audio data from that output is then internally fed as an input for the app which multiplexes it with video into a combined stream that is then cast to your TV. This is the way you can play sounds from any application you wish.

There is a special chapter on how to download and install the driver here.

Once the driver is installed, it is used on per-session basis, i.e. your current audio output is changed to Screen (TV) output when a session starts and at-that-time-active output is returned once the session is finished. You can change the current output by holding ALT and clicking on repro icon:

or in Sound preferences (selectable from System preferences):