Is Epic Thunder Ground Forces Sound Mod free?

Yes. Epic Thunder Ground Forces Sound Mod is released under Community / WT Live and costs nothing. Neither it nor War Thunder has any paid content.

What do I need to run Epic Thunder Ground Forces Sound Mod?

War Thunder, which is itself free. Epic Thunder Ground Forces Sound Mod is an add-on and will not run standalone.

The PlayBound Launcher installs it in one click from War Thunder's mods list. You can also install it by hand — full steps are on this page.

Roughly 780 MB, on top of the base War Thunder installation.

Epic Thunder Ground Forces Sound Mod — Free War Thunder Mod · PlayBound