I am glad we're not the only ones struggling with this.
Rather than creating a global actor (which sounds a bit scary!) I just created a standalone ak game object, with itself as a listener, to achieve the same thing.
However, there was one extra bit of logic necessary to get it to work, which took rather a long time to track down, which is that (with our setup, at least) it is necessary for there to be a spatial audio listener active for audio to be audible. During level transitions there is no spatial audio listener in the default UE4 plugin. I got around this by setting my aforementioned standalone ak game object as the spatial audio listener at the point where the regular spatial audio listener is removed/destroyed.
Hopefully this helps out someone else in the same situation :)