I’m just guessing, but I think it means that they are currently out of stock, an order is in transit, and a unit in the shipment is reserved for you. I can’t guess what the transit time might be.

Thanks.  That makes sense.  

While I don't work for Sonos, I do work in the fulfillment sector. Often times we use allocation to mean that your order is amongst the amount we have available to ship. 

For example, at my job: We have 300 units. We have 350 orders that came in. The allocated orders are the first 300 orders we received. The last 50 orders would be put on a "backorder" status. 

Maybe it's something like that.