This could be caused by iOS interrupts not being handled correctly by the ad mediator.
A solution could be to manually call
FMODUnity.RuntimeManager.PauseAllEvents(true);
FMODUnity.RuntimeManager.CoreSystem.mixerSuspend();
just before your ads play, and then resume operation with
FMODUnity.RuntimeManager.PauseAllEvents(false);
FMODUnity.RuntimeManager.CoreSystem.mixerResume();
when the ads finish.