Here are a few thoughts, but not (yet) a specific solution.
You might try a "wireless music system" type of consumer audio or PC accessory.
I think that companies like Logitech and others make wireless accessories that take an audio output from either a hi-fi (via RCA plugs) or PC (via USB) and transmit it over either 900 MHz or 2.4 GHz to a receiver that has an audio output. I have an old Logitech unit, although it's advertised range is only 330 feet. I am sure that here are others out there.
I also recall seeing units by Linksys and DLink and Slim Devices in places like Best Buy or online that use Wi-Fi to link a PC to a stereo.
There may also be some sort of Wi-Fi solution. I have seen articles on the internet of people building inexpensive yagi antennas for Wi-Fi to achieve amazing ranges that are much longer than your application. Someone published a how-to article of building one inside a Pringles can.
If you want to go with a wired or cabled approach, you could use cheap Cat 5 cable and a pair of baluns that take stereo audio for a pair of RCA plugs and convert it to a single RJ-45 plug. The same balun at the other end converts back to RCA output. I recall seeing that for about $100 and I recall the range was at least a couple of thousand feet. Here's a link for one that I just googled:
I have no idea whether it is any good or whether the dealer is reputable or not, but it illustrates the concept.
Here's a long range Bluetooth device:
Again, I can't vouch for these, it's just something I googled or heard about.