Hi, how can I jam with my guitar or bass to a track in e.g. youtube?
If JACK or ALSA is used by ardour, all other sources are muted.
a) use your phone to play youtube, and let the PC do ardour!
b) download sound (many youtube to mp3 web services…) from YT and import into ardour
there are also options to connect your browser as another jack client, but I wouldn’t consider it.
Is there no comforable way with Linux?
In AV Linux, the web browser can be a Jack client but I’ve never tried using it that way.
In kxstudio alsa2jack bridge provides a seamless integration. All non jack applications keep working like before
/Anders
youtube-dl fetches the youtube clip to disk and vlc with jack support plays it if you don´t want to bridge between alsa and jack.
Or if you use Firefox download extension “video downloadhelper”, it also lets you download Youtube videos. Play these with VLC. It’s quite handy 
You do not mention which linux distribution you are using, and whether applications typically use pulseaudio or access alsa directly. A large number of distributions currently use pulseaudio, and if you are using a recent version of jack2, pulseaudio and jackd work pretty well together. On my fedora installation when I start jackd pulse loads the jacksink module and all system audio goes through jack unless I specifically disconnect the pulseaudio connections. That setup is definitely the easiest to use.
Are you using Linux? Check this page. http://alsa.opensrc.org/Jack_and_Loopback_device_as_Alsa-to-Jack_bridge. You can create an alsa loopback device in JACK so all alsa clients are routed to JACK.
I don’t know what system you’re using but, in my opinion, and maybe this is stupid, the easiest way to do this (unless of course you just install KX or AV where it’s already configured) is to boot the KX dvd, grab the .asoundrc file out of the home directory, boot into your system,drop the .asoundrc into your home directory and reboot. You should be good to go. You might want to install Cadence if you need to manually start the bridge. Also, the snd_aloop module needs to be built into your kernel, which it most likely is. This worked for me in Arch, but as I said, the simplest way would be to install AV or KX. No one should have to play YouTube on their phone to jam. I frequently record the audio from karaoke videos on YouTube directly into Ardour for the kids and they record their vocal tracks. It’s easier than downloading the video, converting it and importing into Ardour. I think Ubuntu Studio has a PulseAudio to Jack bridge with similar results. It’s definitely do-able with not too much effort.
I’m not at home right now but I would post the contents of my .asoundrc if you would like.
I guess it says what to put in the .asoundrc on the alsa page I linked.