When you buy a cheap second hand mini PC to use as a media player in your living room, you can install KODI home theater software on it and it will work much faster compared with the use of a Raspberry Pi.
See:

https://kodi.tv/

Using an older mini PC has the advantage of a low price but could also have some disadvantages...

For example when it is provided with the older HDMI version 1 it could happen that audio is not supported via the HDMI connection, but only via a separate 3.5 mm jack connection.
This results in manually switching (plugging and unplugging) between your TV audio and the media player audio which becomes very frustrating after a while.

To overcome this situation you can use a so called 12 Volt infrared remote controlled relay, see:
http://s.aliexpress.com/2Y3eiaMv

It is a very inexpensive relay, provided with an infrared sensor that switches the relay ON/OFF when it receives a certain code signal via the remote that comes with it.

So with this little relay you can switch the audio between your TV and the media player from your couch with this remote. Great.

But you can forget about this aditional remote and fully automate this control when you use a programmable remote, for example a Logitech Harmony, see:

https://www.logitech.com/nl-nl/harmony-remotes

Just add this relay as a component to your setup and switch it ON when you start watching content from your media player and switch it OFF when using your TV.

In the case described here both inputs (the TV & the media player) are one by one connected to the same output, a soundbar and a set of wireless headphones.

However one small addition must be made to the original relay:
The relay has only one switch-over contact (a normally open and a normally closed contact), this means you cannot switch a stereo signal. This signal needs two change-over contacts, one for the left channel and one for the right channel.

So I had to add one additional relay with two change-over contacts.

Step 1: Components

1 little plastic black box that nicely blends in with the rest of your media equipment

1 piece of PCB with holes 0.1 inch and copper islands that fits inside the box

2 indication LEDs (one red for OFF and one blue for ON)

2 resistors 10 kOhm for the indication LEDs

1 12 Volt DC relay make Schrack type RT424012 with two change-over contacts

1 suppressor diode IN4004

1 12 Volt remote controlled infrared relay, see: http://s.aliexpress.com/2Y3eiaMv

1 12V DC power supply minimum 200 mA.

1 female plug for the power supply connection

10 soldering pins to connect the internal wiring

Step 2: Build It Together

Make the PCB fit into the box. I used the guiding inside the box to fix the PCB without bolts.
Mount the Schrack relay, the resistors and the diode on the PCB together with the soldering pins and wire them up as shown.

The infrared relay can be bolted inside the box with one bolt M3. Make a hole in the front side for the infrared sensor and also for both indication LEDs.

About these LED's: Don't be over-enthusiastic about the lighting level of these LED's... Maybe you will need to change the resistors from 10 kOhm to a higher value. These LED's could be very annoying when you are blinded by their bright light... Just test them in the evening and change the resistors to a value that fits your situation.

Make two holes in the rear side for the cables coming from the audio connections from the soundbar, the mini PC media player, the TV and the wireless set of headphones.

Mount the female plug for the power supply in the middle of the rear side. Wire everything up as shown in the diagram. Connect the left- and right channel wires to the contacts of the Schrack relay as shown in the diagram and connect their ground connections together.

Do NOT connect this ground connection to the ground connection of the power supply. This keeps the switching control circuit fully separated from the audio signal wiring. Which is safe.