Same file plays differently (x2 speed or chipped sound) depending on which server it comes from

Plex Players Gaming Consoles
1

Hi!
I have an 1080p mkv with 5.1 AAC sound on two servers on the same wifi: an iMac and a MacBook Pro. On the X360, it plays fine from the MacBook Pro, but with x2 speed from the iMac (the sound is sped up too, but smooth). I tried to read the file on my iPhone, and again it plays fine from the MacBook Pro, but with a buggy sound when read from the iMac (at normal speed though).

Anyone has a clue about this?

Thanks for your help!

PS: Thinking about it, the other difference between the two Macs except for their model is their OS: Sierra for the iMac, El Capitan for the MacBook Pro.