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Question: Can the transmitter broadcast to multiple receivers (1 to many)? Can you get an extra receiver seprately?
Document: The only 2 issues I noticed are:1) Works best with line-of-sight connectionSince the receiver is facing the transmitter at a distance of 20 feet, I never had any issues with breakups. However, during installing the kit I noticed that when I placed the receiver behind the speaker then I would get sporadic breakups. So this unit is probably not going to work well through walls.2) Tends to interfer with other wireless gearThis kit uses a spread spectrum signal at 2.4 GHz.
I bought my second module to do a Zone 1/Zone 2 configuration with my receiver. I used one set to set up the back speakers in my living room and then used the second set of receivers to set up speakers in my kitchen.
WEM works well as a wireless device but cannot expect the sound from the use of a wired system. The system works well in provinding  those extra speakers in the master bedroom(20x15) with out triping over wires and without too much  ghost signal interference. When I expand the distance between  the WEM transmitter and  receiver there seems to be an increase in signal noise.For critical listening and home theatre the the hard wired sound room is used.
I suggest anyone thinking of buying this consider the number of people who couldn't get it to work. I'm among those and I wish I had heeded my own advice. The transmitter and receiver were 12 feet apart and they wouldn't link up; I tried every channel. I was trying to connect a surround speaker where it wasn't convenient to run a wire.
After a lot of fiddling with the channel controls, I was finally able to synchronize the wireless receiver with the JBL transmitter. However, the sound drops out completely every 10 minutes. In addition, every few minutes, there is a popping noise and some static. I have spent over one hour trying to get this to work but now I finally have given up and am returning the set.
I like to do tons of research and find the best items I can get for the money. Hopefully this helps somebody out.Setting up the speakers and WEM-1 was straight forward and gave the receiver no issues when calibrating the speakers. Just to be clear, the WEM-1 keeps me from having to run speaker wire from one side of the room to the other.
The system also has a nice feature that it powers on on-demand and goes to stand-by after 10 minutes if no music is played.I could not get it to work using the line in connection. It could be that my receiver out signal is too low or the JBL WEM-1 input sensitivity is too high. Either way I could not get it to work this way, but since this type of  use was not for my setup I didn't mind.
The JBL's WEM-1 50-Watt Wireless Amplifier and Expansion Module is easy to install and perfect for my huge backyard.I paired the WEM-1 with JBL's Control Now Bookshelf/Wall Mount Outdoor Speakers to get PC internet radio to our shop. The system was very easy to setup.The remote is handy since I mounted the receiver 12' up on the shop wall to avoid damage.
I bought this unit to control rear surround-sound speakers in my living room, where I did not want speaker wires strung along the floor. I wired the transmitter to the rear surround speaker outputs on my Yamaha 5.1 receiver, so the rear speaker volume would be controlled at the receiver. Zone 2 applications would, of course, have different requirements, and the unit does come with a remote to control the volume.
After fine tuning the levels on my receiver I can honestly say that I can tell no audible difference in a wired setup and the wireless setup that I now have.The build quality is also exceptional. My initial reaction was "WOW, these are really well made!" You know from the instant you open the box that both the transmitter and receiver definitely aren't constructed of cheap plastic as some other wireless systems are.Overall, I am very pleased with the JBL WEM-1 and its performance thus far.
Answer: The transmitter can broadcast to up to 4 separate receivers.