Changes your Cat or Dog into BB8 depending on which pet u picked.u can use both cat and dog file or just use the 1 you want to replace.Also doesn't change audio so BB8 will sound and behave like a Cat or a Dog depending on what u picked in game.

EDIT: The new versions has a Wave Bank file that adds the BB8 sounds, simply copy/paste it into the XACT folder in Stardew Valley\content. If you rename the original (i.e. Wave Bank_OLD) you won't have to overwrite and can easily restore to the original.
You can choose between the default soundtrack and the one modified by Drogean called More Music Mod, both will have the BB8 sounds and pictures, the latter simply has added music to some parts of the game that were silent.