Hey Araklaj,
So since you are working on an engine version that is more than a year old, it is going to be hard to determine whether your issue has already been fixed, and how to troubleshoot.
The only thing I can say is to try to test between standalone and packaged build. Then look at the logs to see if there are any outstanding references to FMOD not changing the volume etc.
If that does not work, you might need to find an alternate route to modifying the volume levels instead of a slider.
Cheers,