Ok, I found the cause of my problem. The sounds were occluded by the charater the are playing on.
I fixed it quickly by adding the Actor owning the audio source Component to the list of actors to ignore (which contains the player controller Pawn)
In void UAkComponent::CalculateOcclusionValues(bool CalledFromTick), replace:
bool bNowOccluded = GetWorld()->LineTraceSingleByChannel(OutHit, SourcePosition, ListenerPosition, ECC_Visibility, FCollisionQueryParams(NAME_SoundOcclusion, true, ActorToIgnore));
by:
FCollisionQueryParams CollisionQueryParams(NAME_SoundOcclusion, true, ActorToIgnore);
CollisionQueryParams.AddIgnoredActor(GetOwner());
bool bNowOccluded = GetWorld()->LineTraceSingleByChannel(OutHit, SourcePosition, ListenerPosition, ECC_Visibility, CollisionQueryParams);