@joys of sight
Hearing the ambulance (or police car) converging from the next block tells me I may need to move over (this is London - emergency vehicles often pass wrong side of traffic islands etc.
Thanks for the info! Perhaps we have it worse here, as traffic on the wrong side of islands is not limited to the emergency services...
But the crucial point is - do you move over and stop before you see the ambulence?
When I'm in the car (or on the bike), whether I hear a siren or not I wouldn't do anything differently until I had established where the vehicle is and where it needs to go (by seeing it). Otherwise, for example, I might pull into the bus lane and end up obstructing an ambulence coming up it. Or vice versa.
I don't think it can be denied that hearing an ambulence at least prepares you for the possibility that you might need to manouvre, but I'm not convinced that it is giving you anything more than a sort of comfort blanket. The exact same argument applies to in-car radios, to convertibles with the roof up (or any car which doesn't have its windows down). And of course, in most urban environments the average speed of a cyclist is as high (if not higher) than motorists, so there's no real argument that they don't need to hear because they are going faster or whatever. The reason they don't *need* to hear is simply because they don't *need* to hear.
(And for what it's worth, I quite like having the windows down when I drive in town, because it's sometimes nice to hear what's going on. I don't feel unsafe when I roll them up & put on the radio though!)