SFML-app | Just messing around with sfml | Reactive Programming library

This rule doesn't create the file it promises to:

By defining the same class differently in different places, your program violates One Definition Rule:

[basic.def.odr]/5 There can be more than one definition of a class type ... in a program provided that each definition appears in a different translation unit, and provided the definitions satisfy the following requirements. Given such an entity named D defined in more than one translation unit, then

— each definition of D shall consist of the same sequence of tokens; and...

Emphasis mine.

Remove class definition from Game.cpp, only leave member function definitions, like this:

There could be many reasons why no sound is played, even if your code was entirely correct.
However, it isn't, as sf::Sound::play asynchronously plays the sound in another thread (as stated here) but your application finishes pretty fast, thus giving the other thread no real chance to play much of the specified sound buffer.

So in order to have at least a theoretical chance of hearing anything, I would suggest to insert some instruction to block the main thread, like