I don't have direct experience with wireless speakers, but I can offer the following reasoning on why you should prefer wired speakers:
Most of the frequency spectrum is allocated to licensed users. Products like wireless speakers, bluetooth headsets and WiFi run in the unlicensed ISM bands of 900 MHz, 2.4 GHz and 5.7 GHz. There isn't a lot of bandwidth to begin with. The situation is made worse with lots of WiFi networks, bluetooth mice and various other gadgets. In a meeting room situation, all these devices are likely to be present and competing for limited bandwidth.
Wiring up your speakers will cost more, possibly a lot more, but it is an expense that you can amortize over the entire lifetime of the conference room. Wireless may be cheap, but the frustration of having the speakers go out during a critical presentation might overrule the initial cost savings.