The signal may be a little weaker and the background noise a little stronger. But if you need 10m, then buy 10m.

I need at least 12 metres for my project. Is there a longer cable somewhere? Or do I have to put together a 10-metre and a 3-metre cable? And if so, can I still listen to music comfortably with it? (Maybe enormous noise).

Well, if you need 10m, you need 10m ;)
I would say you can't hear any difference. However, some people may have a different opinion.
Maybe you can hear a difference with super expensive, high quality professional headphones.
Audiophile connoisseurs will set me straight.

Honestly, I would buy a 10m cable without a second thought. In the worst case, you exchange the product and buy a 5m one.