my 2 cents.
if you are asking how wireless transmitters work, you should most definitely not be building one you should be taking a class at a school about it or something, THEN try building one which wont be FCC compliant and can get you arrested in Europe which actually happened to a good friend. We have FCC laws here which arent really enforced on the ham level but in europe its a much different thing, friend of mine had army dudes haul him off for transmitting on their band...
I use audio transmitters at work quite a bit and work with other guys that do not have cables in their package, everything is wireless. People that understand how to make them dont really know how to make them sound good. If you are really into learning about radio by all means go for it, but if you are looking at this as a utility item to use for your guitar or something, completely just buy one from ebay. The sennhieser units arent so bad for a guitar signal, you'll hear the companding with a mic however on all those cheaper units. A "real" wireless system doesnt cost a small fortune for no reason.
dave