Step 1: Things You Will Need
A video transmitter and receiver. The lower the frequency the better. I am using one that transmits at 1080 MHz. I purchased the transmitter on its own as I already had a receiver from a wireless camera which burned out because I thought 30v would give more power.
Two coaxial cables to connect everything
Power supplies or batteries
An spdif to analogue converter I got one for £11
Earphones or an amplified speaker
A computer or other device with a digital audio out coaxial connection
Step 2: Connecting All the Devices
Plug a power supply into the transmitter. I use a 9v one because 12v makes it heat up too much.
Plug one end of your other coaxial cable into the coaxial input on the converter box and the other into the video output on the receiver. The converter box I have is 5v and the receiver is 9 to 12v.
Plug earphones or your speaker into the audio output on the converter box and then plug in the power for the box.
Plug in the power for all devices and play music on the stereo. If you are using a computer with digital output, you need to go into your playback devices and select your digital output. You also need software where you can select the playback device I have Realtek digital output. I have it set to 2 channel, 24 bit 44100Hx studio quality. This lets me get to the other end of our old farmhouse through thick walls before the signal starts to break up. If the signal breaks up, select a lower quality setting.
Step 3: Tuning In
Although these little transmitter kits appear to be designed for video only, they are actually just very wideband radio equipment which can be used to transmit anything you like. The lower the frequency, the better as you will get better range through walls with lower power. Do not go for the 2.4 and 5.8 GHz rubbish as those are really only useful for FPV on model aircraft.
Another interesting use may be a high speed serial link if you were to connect a USB to TTL converter to the video input of the transmitter and on the receiver end, you would have an amplifier to bring the signal up to +5v so the other USB to TTL converter could decode the signals.