@mahks
To expand on above explanations. If the game located in C:\username/triplea/downloadedMaps/*gamename*/map is lacking an image or sound file. It will look in the following location. Which is the TripleA engine's installed location.
The typical install location for the triplea engine and the "assets" folders are below.
C:\Program Files/TripleA v.1.9.x.x.xxx/assets
flags
misc
resources
sounds
units
territoryEffects (this one is missing now?)