‘Perhaps’ an  even better/simpler option… just add a Nvidia Shield to your TV - it has audio upmixing to PCM 5.1 audio when using its YouTube App. Sounds quite good on a Sonos Arc HT setup.

Thanks for your fast reply.
Well, my actual problem is I'm using a HDMI switch of FeinTech VAX04101. Where I'm also connecting my pc. Do you have any other suggestions for this setup?

Nice pass-through switch (I want one, but can’t seem to buy those in the UK where I am), but I don’t understand why would that change anything suggested?

https://feintech.eu/en/products/feintech-vax04101-hdmi-earc-pass-switch-4x1

I will try, but they were only delivering to Germany only - I’ve been after one to try it .

When I stream it via Spotify Connect, the music sounds completely different than via the Switch.

As I mentioned before, I guess the Sonos app might be the problem.

I'm also not living in Germany.

I bought the switch from there:

https://www.ksh-technik.de/search?page=search&sdesc=&desc=on&cat=&mnf=&subcat=&keywords=VAX04101

and yes it's the exact same switch, but it was cheaper back when I bought it

I’ve just written to feintech to ask if they will ship to the U.K. so I will see what they say. Thanks

edit: When trying to buy, they only seem to ship to these countries (see attached), but maybe they will make an exception.

That will teach us for leaving the EU..ha ha

@tomSawyer 
Anyhow sorry for taking this thread slightly off track, but if you were to add the Nvidia Shield TV, as mentioned earlier, to one of the feintech ports, then it does have the option to upmix the Youtube sound output to PCM 5.1 - it actually fooled me at one point into thinking Google had updated their App for surround sound audio - till I realised my mistake of activating that option in the Shield’s settings. 

don't worry about that.
This seems to be a solution for streaming youtube via TV, but not for the PC issue.

Do you think an audio software could fix this problem?

I don’t understand, as you made no mention of your PC in your original post and in your later post after that  you said…"if I watch YouTube music videos on the TV, and they don't come in music stereo”…No mention of your PC?

Anyhow if you’re referring to the sound-out from your PC - then ‘yes’ it might be the App on the PC or your sound card/drivers/codec perhaps? Do other PC apps provide stereo audio output? ..if so, then it’s likely the YT App itself.

I’ve managed to order the VAX04101A today from Germany to ship to the U.K. - thanks @tomSawyer. Just need to dig out an EU adapter for when it arrives.

@Ken_Griffiths you are welcome