The challenge is the squeeze from airfoil, roll your own, or just moving your ipod into the other room on the low end, and Sonos on the high end.
Sonos can always drop their pricing and produce a Sonos Lite for $250 if you become a threat.
Unless you come up with a fundamentally cheaper way to do something, it's hard to justify doing a startup. A fundamentally cheaper way in this case is selling Airfoil -- their marginal cost is $0. They can be more profitable selling Airfoil for $10 than you would be selling AlmostSonosLite for $200.
The other problem is user adoption and marketing -- the set of potential users for this is pretty small, relative to the overall consumer electronics market. Convincing people they actually need this is going to be very hard. The main use cases I see for wireless speakers are wireless rears in an HT system to avoid running cable, and temporary setups in large spaces (like at a party outdoors).
The core Sonos use case, listening to a central repository of music or HT audio in multiple rooms, seems rather specialized, and a lot of the potential customers would value the higher audio quality and polish of the sonos system enough to pay for it. If you're not going to put good speakers + amp in each room, having a good quality portable system (like a dock for an ipod) seems like the standard solution.
The biggest indicator that this is a soft market is that Sonos hasn't sold a huge volume in 6 years, even adjusting huge volume for their pricing being kind of high.