1) There are tools to let you stream to airport extremes, e.g. Airfoil.
2) Competing on price is fail. Competing on a fundamental COST differential is sometimes win, assuming the incumbents aren't willing to lower prices to force you out. There is a difference. You surely won't be buying DACs and such cheaper than Sonos, so your only way to be cheaper is to accept lower margins. You could maybe carve out a tiny niche at a lower price such that it is not worth it for Sonos to lower their own prices (for fear of lowering their total profits), but I know I would rather be Sonos selling at $800 than a competitor selling a smaller volume at $250.
3) A $50 BOM is probably a $250 product. Also, in hardware, going from $50 BOM to $150 BOM (or, going to ipod-level volumes) lets you buy MUCH better DACs and such. Plus, industrial design.
The only opportunity I see is for an "embeddable sonos standard" where you actually eliminate the device entirely, and are built into AVR or TV. However, that is a lot easier for a big CES vendor than a startup. Or, something Sonos themselves could do.
Competing just on price (within a fairly narrow window, like less than 10x) seems like a fail. $100 BOM turns into a $200-300 product. If a $300 product came out, Sonos could easily come out with a feature-reduced version of their product.