Whenever I started a campaign as Parthia there wouldn't be any music on the strategic map. I looked everywhere online but could no find anything. In the end I checked the files out to see if there be something missing or a typo and guess what? tadaaaa, I found that in Play_Parthia\data folder open descr_sounds_music and scroll down to Parthia CAMPMAP section the 3rd line bellow that is a "folder data/sounds/musicb". Just delete the "b" at the end of that line. Restart game and Parthia campaign music is back. This worked for me anyway, i could be wrong.