How to connect Sony headphones to Avantree transmitters and dongles?

Before pairing, please note that most Sony products currently do NOT support aptX-Low Latency codec, hence there will be lip-sync delays when used to watch TV/Video. You may utilize an aptX-LL Bluetooth receiver to eliminate this issue.

Recommended Products: Avantree AS70, Avantree Roxa Plus.

Step 1: Get your Sony Headphone into Bluetooth Pairing Mode.

Different products tend to have a different-looking pairing mode... but not to worry, see below for the pairing mode for a list of Sony's most popular products.

• Sony WH-1000XM4, XM3 (and XM2): To enter pairing mode, make sure the headphone is OFF. Press & hold the power button for 7 seconds until you hear a "Blueooth pairing" voice prompt. The LED indicator will flash BLUE & RED for XM2 and double-flash BLUE for XM4 and XM3.
  
• Sony WF-1000XM3: Put the earbuds on, press & hold the touchpads on both earbuds together for 7 seconds. You'll hear a "Blueooth pairing" voice prompt.
  
  
• Sony WI-1000X: This headphone supports aptX and aptX-HD. To enter pairing mode, make sure that the headphone is turned OFF. Press & hold the power button for 7 seconds. The LED indicator will flash BLUE & RED and you'll hear a "Bluetooth pairing" voice prompt.
  
  

Step 2: Put your Avantree Bluetooth transmitter into Pairing Mode as well.

Please refer to its user guide if you're not sure how. With both devices in pairing mode, keep them close to each other and give them a few seconds. Please check their respective LED indicators to make sure that they've connected.

In case you prefer Learning through Videos:

If no luck to get them paired, please Factory Reset/Clear pairing history of both devices and try again. 
How to Factory Reset SONY Headphones