How can I listen to multiple audio outputs at the same time?

Question

I want to practise playing piano with my Sony WH-1000XM4 headphones while listening to music on YouTube, so that I can practise playing as if joining in with other musicians.

Now, the headphones work over bluetooth. That means I can listen to YouTube. however, the headphones cannot simultaneously accept the 3.5mm audio from the piano. I want to be able to listen to both. I can tolerate an audio delay from the PC but obviously not from the piano.

This leads me to the idea that I can mix the output. Maybe feed a 3.5mm input from the PC and the keyboard and feed the output as 3.5mm into the headphones?

What sort of equipment do I need to mix the audio? How about something like this - is it overkill?

https://www.amazon.co.uk/Maker-hart-Just-Mixer-Portable/dp/B0824Q3WY8/ref=sr_1_5?keywords=audio+mixer+3.5mm&qid=1637233099&sr=8-5

https://www.amazon.co.uk/ALTO-ZMX52-Professional-5-Channel-Compact/dp/B004TM31QK/ref=sr_1_26?keywords=audio%2Bmixer%2B3.5mm&qid=1637233099&sr=8-26&th=1

Answer 1

Since your headphones can accept both Bluetooth and wires, and Blutooth is not made to play the piano (even so called « low latency » codec like AptX are made to watch movies, but reduce the latency to 40ms which is still too important), the only way is to mix the piano and YouTube and send the mix by wire to the headphones.

https://vb-audio.com/Voicemeeter/index.htm Can help to create a virtual mixer. (The idea is to link your piano to the line input of a desktop PC, then the PC to the phones. A low latency driver like ASIO4ALL can be useful).

Beware, some (most) mixers output line levels and are not powerful enough to driver headphones : phones output is needed. Both mixers you point have a phones jack, then you can choose them.